"We are very concerned about the various measures that are being taken by the authorities to restrict freedom of expression in Malaysia, including preventive detention and the government's decision to reportedly declare Bersih an unlawful organisation," said Rupert Colville, spokesman for the UN High Commissioner for Human Rights.

Electoral reform group Bersih is organising a July 9 rally which they have said would be peaceful.

But police have arrested about 150 opposition politicians and activists over the past week for trying to drum up support for the rally. Most have been released after questioning.

Provisions such as the Internal Security Act and Emergency Ordinance allow police to detain activists, and police have said that the planned rally is illegal as it has not been granted permission.

"We call on the authorities to release all those being detained for peacefully exercising their freedom of expression and urge the government to take steps to repeal preventive laws," said Colville.