About 300 people, according to PKR estimates, had converged on motorcycles and demanded that PKR withdraws its support from the Bersih 2.0 march on July 9.
PKR secretary-general Saifuddin Nasution, who rushed to the HQ after being alerted by party members, said he arrived at about 11pm and observed the demonstration from a distance.
"How do I know they are Umno members? First of all, they were carrying Umno flags and they were wearing their red 'Patriot' T-shirts,” he told Malaysiakini.
“I could also see (Umno Youth exco Lokman Adam) and (former Perkasa Youth chief Arman Azha Abu Hanifah) leading the group."
Saifuddin (right) also said that when he got there with several party members, they recorded the fiery speeches made by Lokman to the Umno Youth crowd.
"He said that he wants to burn down the PKR HQ. He can deny it all he wants, but we have the recording," he said.
PKR Youth deputy information chief Rozan Azen Mat Rasip, who was also there, confirmed that Lokman had explicitly threatened to burn down the headquarters if the party participates in the July 9 rally.
"He also threatened (PKR de facto leader Anwar Ibrahim) and (Bersih 2.0 steering committee chairperson) S Ambiga for supporting the rally," he said, adding that about 50 PKR Youth members were there to monitor the demonstration.
According to Rozan (left), the Umno Youth group gathered in two locations - at a petrol station in Shah Alam and at the Batu Tiga toll plaza on the Federal Highway - before making their way to the PKR HQ in Tropicana. This was based on information in pro-Umno blogs.
He lodged a police report at the Tropicana police station, just a few doors down the road from the HQ, at about 11.40pm, almost immediately after the crowd had dispersed.
No arrests made
Rozan said he saw about 20 heavily-armed police officers on standby at the protest although no arrest were made.
"That's why we're puzzled," he said, urging the police to investigate the case under criminal intimidation.
Lokman and Arman could not be reached for comment.
Though Bersih 2.0 - which stands for the Coalition of Free and Fair Elections - claims to be a politically non-partisan group, PKR is expected to mobilise tens of thousands of its members from around the country for the massive rally.
In calling for electoral reforms, Bersih 2.0 intends to hand over a petition to Istana Negara detailing their eight demands before the next general election.
However, Umno Youth has said that it will also organise a counter-rally on the same day, calling it the 'Patriot' rally, in support of the current electoral system.
Movement chief Khairy Jamaluddin had previously said that their objective is to defy Bersih 2.0, as well as to support engagement with the Election Commission.
Malay rights NGO Perkasa has vowed to hold a counter-rally in a bid to stop the Bersih rally.