Police said they may pursue sedition charges against speakers at a mass rally organised by Malaysian opposition leader Anwar Ibrahim to protest against alleged election fraud.
The announcement was swiftly condemned by Anwar's party as a "politically motivated threat" aimed at silencing opposition claims that last Sunday's hard-fought elections were stolen by the ruling coalition.
Vowing to "never surrender" as he addressed tens of thousands of supporters at a rally in a Kuala Lumpur suburb, Anwar on Wednesday announced that a series of protest gatherings would be held around the country.
Rafizi Ramli, an official from his party, said two rallies were planned for the weekend in northern states including Anwar's home state of Penang, with another two early next week on the east coast and in the south.
During Wednesday's rally, at which supporters dressed in black to protest against the election outcome, Anwar denounced the National Front government of Prime Minister Najib Razak as "illegitimate".
Hisan Hamzah, police chief of the state of Selangor where the rally took place, said he may pursue sedition charges against most of the 33 speakers at the rally. He would not confirm whether Anwar would be among them.
"This is a very serious offence," he said. Sedition carries a penalty of up to five years' jail.
A statement by Anwar's party said: "This illegal police investigation is clearly intended to divert attention from the massive electoral fraud and irregularities that are being highlighted by Pakatan Rakyat (People's Pact)."
The huge turnout and the charismatic Anwar's call for similar rallies across the country upped the ante in an opposition campaign to paint the elections as a fraudulent victory for the coalition that has ruled Malaysia for 56 years.
Anwar has said he would soon produce evidence to prove his claims.
Najib has denied the allegations of fraud and urged opponents to accept the election result gracefully.