The rally - dubbed Himpunan Hijau 2.0 (Green Gathering 2.0) - seeks to gather 20,000 people from all over the country at the Kuantan Municipal Council field.
Pakatan Rakyat's top leaders have also pledged to attend the event, in protest of the Lynas Advanced Materials Plant (Lamp).
In a nod to the newly passed Peaceful Assembly Act 2011, the police have sanctioned the rally, albeit at the eleventh hour, and imposed 12 conditions. Rally organisers have agreed to the conditions.
The two-hour event is suppose to kick off at 10am.
Live reports follows:
8.32am: Almost 1,000 people, mostly clad in the symbolic green, have gathered on MPK4 field and more are trickling in.
Some activist from out of state are gathering at several nearby hotels.
The mood is jovial at the entrance of the Megaview Hotel where Bersih 2.0 leaders are leading a crowd in singing Suara Rakyat.
National laureate A Samad Said, who is scheduled to recite a poem later, is also spotted in the crowd.
8.55am: The Megaview Hotel crowd being led by Bersih 2.0 leaders are marching towards MPK4 field.
They are mostly clad in Himpunan Hijau 2.0 t-shirts or green coloured garbs to symbolise their environmental cause.
Some of them are holding placards and banners to protest against the Lynas plant. Yesterday, the police said they would not bar them from doing so.
8.58am: Festivities is already underway at the MPK4 field with performance group Theatre Bukan Theatre is staging a sketch mocking Pahang Menteri Besar Adnan Yaakob.
"As menteri besar how much will you offer me for your factory?," said one acter.
Another man, dressed to look like an engineer with the words "Lynas" across his back responded: "RM700million!"
"Wow! I accept! I'm going to London," replied the 'menteri besar'.
9.00am: The crowd is swelling at MPK4, estimated at 1,500 with the latest group of balloon-wielding protestors.
Police presence appears moderate, numbering at about 100, backed by two sniffer dogs. A police helicopter was seen hovering near the field.
9.10am: While it is bright and sunny at Kuantan, it is a wet and soggy experience for some 30 activists who are holding a solidarity rally under the rain in Jalan Song, KUCHING.
This is one of several solidarity rallies that are simultaneously being held in several major towns across the country.
9.13am: About 1,000 people are marching from the Kuantan wet market towards MPK4 field. This is the biggest march thus far.
This crowd is proceeding towards the field quietly but they are met by a cacophony of horn blaring and cheers by passing vehicles.
9.15am: The crowd at the MPK4 has easily grown beyond 2,000 people. It is a festive atmosphere with groups chanting slogans sporadically while others stage various performances.
9.20am: "Stop Lynas" and "Hancur (Destroy) Lynas" are popular slogans.
9.30am: Stop Lynas Coalition chairperson Andansura Rabu has arrived at MPK4, back by some 500 Badan Bertindak Anti Rare Earth Refinery (Badar) and PAS activists. With him is PAS vice-president Salahuddin Ayub.
This group is chanting: "We want Lynas to close down, this is to guarantee our health"
9.40am: The crowd size is now estimated at 5,000 as more groups march in.
The scorching heat is causing many to sweat profusely.
9.50am: At a solidarity gathering in Kuala Lumpur, some 200 people are gathered at Maju Junction dressed in green.
"Please oh God, Allah please help us fight against the destruction of our planet, destroy the corrupt, in the name of God we hope the event today will be a success." The event is now officially underway.
Police's crowd figures via Twitter: 4,000 - 5,000 as of 9.45am.
10.05am: Over at KUALA LUMPUR, Ayam Fared recites 'Gebeng yang luka', a poem by A Samad Said for the crowd.
10.08am: Hishammuddin is emceeing the event and have urged the crowd to be seated. He estimates the crowd to be around 10,000 people.
He leads the crowd in singing Suara Rakyat, before a Chinese drums performance.
10.10am: Poor sound system makes the stage hard to follow for most people. Volunteers are criss-crossing the field to advise participants to sit down.
"Should we gather money now to burden our future generations?"
To this, the crowd responded with an emphatic "No!"
"Then what should we do?" to which the crowd responded: "Hancur (destroy) Lynas".
He then urged the public to fight in the struggle against Lynas in order to safeguard the environment for their grand children.
10.15pm: NGO and Pakatan Rakyat leaders are being brought on stage. Among them is Kuantan MP Fuziah Salleh, who has been Pakatan's most ardent campaigner against Lynas.
By most accounts, the crowd at MPK4 is nearing 10,000.
10.17am: Other environmental groups are also using Himpunan Hijau 2.0 as a platform.
A group of activist from Bukit Koman, Raub have taken the stage to tell the crowd of their struggle against gold mining activities which uses highly poisonous cyanide.
They hope that the authorities will realise the life is more important that profit.
“We don't just want 1Malaysia, we also want a 1Healthy Malaysia,” said a representative, which drew rapturous applause from the crowd.
10.20am: DAP veteran Lim Kit Siang arrives and is greeted by the crowd.
10.30am: A check on Lynas Corporation's website reveals that it has been defaced by hackers.
The hacker who calls itself HackerXL has replaced the website with another which reads "Stop Lynas, Save Malaysia. Do not hurt my country".
A fluttering Malaysian flag is seen in the background.
10.40am: Other environmental groups are addressing the audience, including an anti-incinerator group from Cameron Highlands, Pahang and a group opposing the construction of high tension electrical pylons in Kampung Baru Rawang, Selangor.
10.45am: Bad audio system means that most protestors can't hear speeches. Many make do by posing for pictures and having fun on their own.
10.50am: Anwar Ibrahim arrives and is given an ovation by the crowd.
He shows up with a black jacket which hides a Himpunan Hijau 2.0 t-shirt.
In KUALA LUMPUR, the crowd at the solidarity rally outside Maju Junction has swelled to an estimated 500 people.
10.55am: The crowd is tickled by the appearance of a BN umbrella among them. The owner of the umbrella is believed to be a local MCA member.
10.57am: Audio system is improving at MPK4, allowing more people to listen in. For those who still can't, they come up with activities of their own to cause a carnival like experience. There are small groups seen singing and giving impromptu speeches.
11.00am: A representative from Himpunan Hijau 2.0 takes the stage and reminds the audience that Pahang Menteri Besar Adnan Yaakob had once said he would show up at the rally.
The representative adds that the rally today showed that Malaysians can hold protests peacefully.
Ambiga said that those who make decisions on the Lynas project stay very far away from the Gebeng Industrial zone, which will host the rare earth refinery.
She ask the people to reject the potential lawmakers who support the Lynas project and those without integrity.
11.11am: Rally organisers read out the Himpunan Hijau 2.0 resolutions. The three resolutions are as follows:
1) Scrap the rare earth refinery project.
2) Scrap or review all projects which is to the detriment of Malaysians.
3) Ensure that all development projects are in line with the Earth Charter.
11.20am: Dark clouds are threatening to dampen the rally. Some families are seen heading for nearby restaurants for cover.
11:21am: Stanley Low, a 60-year-old mechanic he is here because he was worried for his three grandchildren, who are all under the age of three.
"I'm a Kuantanese, of course I must care about this."
Asked if he felt this would make any difference, he replied: "It will be something, if we unite we can give pressure to the government."
11.25am: Emcee Hishammuddin Rais now puts the crowd size at 15,000.
"But there is still one person missing - The Pahang MB," he tells the crowd in jest.
In KUALA LUMPUR, the number of protestors at Maju Junction are still growing, despite it being the tail-end of the rally.
11.27am: Another sextenerian, who wishes to only be known as Haji Mohamad, said he wants to see Lynas Corporation go back to Australia.
"Why do they want to dump their toxic waste here? I will probably die in five to six years but my future generation will suffer because of Lynas," he said in reference to his three grandchildren, the eldest being seven.
“The Australian government and the Australian people must take heed or take concern of the expressions by the Malaysian people.
“We don't want (this project) sacrifice our culture and the safety of the children just because of few corrupted leaders,” he said in front of what organisers claim is a 15,000 strong crowd.
Anwar also stressed that Umno must not turn the struggle against the rare earth plant issue as an racial issue.
After Anwar's speech, organisers declared the rally a success and urged participants to disperse peacefully.
12.15pm: Nanyang Siang Pau news alert says that about 1,000 people joined a solidarity rally in Perak. The rally was organised by local activists involved in the campaign against the now defunct Bukit Merah rare earth refinery.
"This is because their children are not going to stay here. (Prime minister) Najib's daughter is going to be married to a Kazakhstan guy, she most probably will stay in London. (Sarawak chief minister) Taib Mahmud has properties all over the world, he can stay anywhere he wants.
"But we are not going anywhere, we and our children will be staying here," he said to a thunderous applauds.
He urged the crowd to vote against the authorities who are ignorant about public sentiments, before chanting: "Yes to Malaysia, no to Lynas,"
Following his speech, organisers urged the crowd to disperse peacefully.
Reporting by Nigel Aw, Lee Long Hui and Abdul Rahim Sabri in KUANTAN, and Leven Woon in KUALA LUMPUR.