Five leaders of Malaysian Indian activist group Hindraf were arrested on Thursday under controversial security laws that allow for detention without trial, a spokesman said.
Hindraf enraged the government last month by mounting a mass rally alleging discrimination against minority ethnic Indians in Malaysia, which is dominated by Muslim Malays.
Police used tear gas, water cannon and baton charges to break up the street protest which drew 8,000 people, and came just two weeks after another rare demonstration organised by electoral reform campaigners.
Hindraf member S. Jayathas said the five — P. Uthayakumar, M. Manoharan, R. Kenghadharan, V. Ganabatirau and T. Vasanthakumar — were arrested at their offices Thursday.
"Regardless of the arrests this struggle will move on. We have many leaders who are waiting to take over and we will not go backwards but move ahead with the fight for the rights of Indians in Malaysia," Jayathas said.
The Internal Security Act (ISA), which human rights groups are campaigning to have abolished, is currently being used to hold more than 100 people, including about 80 alleged Islamic militants.
It is not thought to have been used against government critics since 2001, when Malaysia was under the iron grip of former premier Mahathir Mohamad who used it to quell a reform movement triggered by the arrest of his deputy Anwar Ibrahim.
Opposition leader Lim Kit Siang condemned the government's move and said that if the Hindraf leaders had committed any offence they should be charged and tried in an open court.
"It is deplorable, the use of the ISA is completely indefensible," he said.
"To resort to detention without trial is a regression to the dark days of human rights violations and is something that will bring further shame to Malaysia's international image and reputation."
Three Hindraf leaders have already been charged with sedition for speeches in which they criticised preferential treatment for Muslim Malays who make up 60 per cent of the population and control the government.
Malaysia is also home to 26 percent ethnic Chinese, who dominate business, and 8.0 per cent ethnic Indians who complain they run a distant third in terms of wealth, opportunities and education.
The Star newspaper said on its website that the two-year ISA detention order had been signed by Prime Minister Abdullah Ahmad Badawi.
Under the legislation — which dates back to the British colonial era when it was used against communist insurgents — the two-year detention periods can be renewed indefinitely.
Abdullah has for several weeks been threatening to involve the draconian legislation against Hindraf, which authorities have accused of having links with Sri Lanka's Tamil Tigers. The group denies the charges.
Malaysia's ethnic Indians are mostly Tamils — the descendants of indentured labourers brought here by the British in the 1800s.
Lawyers and human rights group have warned that the use of the ISA will only inflame the protest movement, which is airing grievances that have been bottled up for many years.
Malik Imitiaz Sarwar from the National Human Rights Society (Hakam) said the government's move was regrettable and unnecessary, particularly as three Hindraf leaders already faced sedition charges.
"The ISA detention will have the unfortunate effect of now stopping genuine attempts on the part of the marginalised Indian community to have their grievances addressed," he said.
Malik Imtiaz said that the opposition figures detained during the "Reformasi" movement of Anwar Ibrahim were mostly held for about two years before being released.