Malaysian police fired tear gas and detained about 180 people on Saturday to break up an opposition-led rally of over 5,000 people protesting against a security law that allows detention without trial.

The protest could put further pressure on Prime Minister Najib Razak whose popularity is under threat after the mysterious death of an opposition politician's aide.

Led by opposition leader Anwar Ibrahim, the crowd of mostly ethnic Malays marched outside the national mosque and at a shopping mall in the city, chanting 'Allahu Akhbar' (God is Greatest) and 'Down with the government'.

A police helicopter circled overhead as police fired tear gas and water canons to disperse protesters who marched towards the palace to deliver a memorandum to the king seeking his support to repeal the Internal Security Act (ISA).

The ISA allows for detention without trial on national security grounds and some analysts say it has largely been used as a government tool to quell political dissent.

'Today we will gather 100,000 people,' Salahuddin Ayub, a senior leader with the opposition Islamist party PAS, told a cheering crowd. 'We want the ISA revoked.

More than 180 people were detained, Deputy Inspector General of Police Ismail Omar said. 'We will keep on arresting until we can shut down this demonstration,' he told Reuters.

Police had earlier closed major roads leading into the city and mounted checks at various places to deter protesters and had warned organisers to call off the protest.

The law was used last year against a pro-opposition blogger, a journalist, and members of Hindraf, an outlawed Hindu rights group, which had brought over 10,000 people onto the streets demanding rights for the country's ethnic Indian minority.

'The secrecy and unscrupulous practice that surrounds the use of the law ... again exhibits the true colour of the ISA,' said a joint statement on Friday by local rights group Suaram, the World Organisation Against Torture and the International Federation for Human Rights.

Twelve people have been detained under the ISA, most of whom are suspected of having links to militant groups including Jemaah Islamiah.

Najib has promised to review the legislation and released 13 detainees on taking office in April.

His approval ratings recently surged to 65 per cent from 45 per cent, according to a survey by independent polling outfit Merdeka Center, after introducing measures to help Malaysia win back investment. – Reuters