Hundreds of police and civilian security officers have been deployed in the Malaysian capital Kuala Lumpur, where protesters began to gather hours before a scheduled mass rally calling for electoral reforms.
Supporters of the Coalition for Free and Fair Elections gathered in an open park in central Kuala Lumpur for Friday's demonstration.
The Kuala Lumpur city government on Friday cordoned off the park after securing a court order to prevent the protest.
Protesters have said they will march to the barricades and demand access but vowed to remain peaceful.
"We will march to the barrier," said Ambiga Sreenivasan, chairwoman of Bersih, an electotal reform pressure group.
Al Jazeera's Harry Fawcett reporting from Kuala Lumpur says: "Already we are hearing that thousands are near Independence Square, there will be a significant turnout."
"The police say they will intervene if people defy the order and cross into the square," said our correspondent.
The mass rally follows one crushed by police last July, when 1,600 people were arrested, and marks a major test for Prime Minister Najib Razak ahead of widely expected elections.
|Malaysian authorities sealed off Independence Square on Friday ahead of the sit-in [AFP]
Last July's rally for clean elections brought tens of thousands to the streets of the capital, prompting a police crackdown with tear gas and water cannon.
A resulting backlash prompted Najib to set up a parliamentary panel whose eventual report suggested a range of changes to the electoral system.
But Bersih and the opposition are demanding a complete overhaul of a voter roll considered fraudulent and reform of an Election Commission they say is biased in favour of the ruling coalition.
The rally is a direct challenge to Najib, who since last year's crackdown, has launched a campaign to repeal authoritarian laws in a bid to create what he called "the greatest democracy".
His ruling coalition has governed Malaysia for more than five decades but made a dismal showing against the opposition in 2008, and Najib is under pressure to improve on that.
Elections are not due until next year but speculation is rife that Najib could call them as early as June.