Kuala Lumpur - Malaysia's opposition alliance on Monday held a massive rally protesting the detention of an opposition lawmaker and a popular blogger under a security law which allows for indefinite detention without trial.

Police on Friday detained opposition politician Teresa Kok and anti-government blogger Raja Petra Kamarudin under the Internal Security Act (ISA), a law drafted almost
60 years ago under British colonial rule to fight a communist insurgency.

Despite the heavy evening downpour, protestors began gathering at a sports stadium in the central Selangor state neighbouring the capital Kuala Lumpur hours before the rally was scheduled to begin.

The rally kicked off with prayers, marking the breaking of a day-long fast for the Muslims during this holy month of Ramadan. Leaders from the three-party opposition alliance were expected to later address the swelling crowds.

Opposition head Anwar Ibrahim was due to give the closing address shortly before midnight.

Organizers have estimated that there will be some 30,000 people attending the demonstration, which surprisingly obtained a police permit.

Under Malaysian law, any public gathering of more than five people must obtain a police permit, or be deemed an illegal gathering. Opposition gatherings and rallies have rarely, if ever, managed to get police approvals.

Raja Petra, who runs the popular Malaysia Today website known for its anti-government articles, was the first to be detained Friday, followed by Tan Chee Hoon, a reporter with the Chinese-language daily Sin Chew. Shortly before midnight, police detained Kok from near her home.

After 18 hours in custody, police released Tan on Saturday.

Police have defended the arrests, saying they had received sufficient evidence linking the two remaining detainees to acts that "were detrimental to the calm, peace and harmony of the country."

"In the interest of the nation's peace and harmony, the police will act without fear or favour. The police wish to reiterate and affirm that at the current moment, the situation in this country is calm and under control," deputy police chief Ismail Omar said in a statement released by the Star online news portal.

Raja Petra, whose website and articles have a large following, was charged with sedition in May for allegedly implying that Deputy Prime Minister Najib Razak was involved in the gruesome murder of a Mongolian woman.

He has been accused by the government in the past of spreading lies and false rumours in his often sensational articles.

Kok has allegedly complained about the noise of morning prayers from a mosque in her electorate, an allegation she has denied.

Lim Kit Siang, leader of the opposition Democratic Action Party, slammed the arrests as "arbitrary, unlawful and undemocratic."

"By labelling Teresa and others as threats to national security without any shred of evidence is a travesty of justice and a gross violation of human rights," Lim had said in his blog.

Critics have long called for the ISA to be abolished, claiming the government was using it to shut out dissenting views in the name of preserving national security. (dpa)