Two PKR leaders were today grilled by police over their role in the PKR de facto leader Anwar Ibrahim's 69th birthday party held outside Sungai Buloh Prison on Aug 10.

PKR communications director Fahmi Fadzil and PKR Youth information chief Lee Chean Chung were probed under the Peaceful Assembly Act 2012.

Before entering Sungai Buloh district police headquarter, the duo took a swipe at the police by holding a placard that read 'Jangan siasat siapa yang menyanyi, siasat and tangkap #MO1!' (Do not probe the one who sings, probe and arrest #MO1!).

The two leaders, who were accompanied by their lawyer Melissa Sasidaran, arrived at 12 noon and were questioned for half an hour.

According to the duo, the police did not tell them which provision of the Peaceful Assembly Act were they being probed.

The two were the first batch of participants of the birthday party to be called in by the police, said Fahmi.

"From what we understand, more will be called. I expect 15 will be called in," he told reporters later.

According to Fahmi, he was probed for being the emcee of the birthday celebration while Lee was called in for singing at the party.

The police asked them for details of the birthday party, including the sound system.

Fahmi said that he never expected the police to summon them over a birthday party celebration.

He added that the police should focus on other more important issues instead of investigating a birthday party.

"I hope the police will offer protection to Federal Territories Minister Tengku Adnan Tengku Mansor so that he can declare his assets without any worries," he said.

The Umno minister has said he would not declare his assets publicly, fearing that doing so would make his family a target for criminals.

Lee, who is Semambu state assemblyperson, asked what wrong had he committed for singing Hong Kong rock band Beyond's 'Glorious Year', a song dedicated to the late South African president Nelson Mandela.

"What's wrong with the song? What is wrong with Mandela? But I am being probed," said Lee.

"We have seen someone probed and charged with painting comics, someone for releasing a yellow balloon. Now, singing is also an offence. Maybe, thinking will be an offence too one day," he said.

"The country will (soon) turn into a huge prison...," he added.

Lee also urged Malaysians to take to the streets on Aug 27 for the TangkapMO1 rally.

Some 500 supporters had gathered outside the Sungai Buloh Prison to celebrate Anwar's birthday.

The gathering was also part of an ongoing campaign to demand his freedom. Anwar has already spent 548 days behind bars.

Also present were Anwar's family members, DAP leaders Lim Kit Siang and Tony Pua; PKR lawmakers Tian Chua, Rafizi Ramli, Sivarasa Rasiah, Zuraida Kamarudin and Saifuddin Abdullah; Amanah deputy Youth chief Faiz Fadzil and Mahfuz Omar of PAS.