More than 500 people have been arrested in the Malaysian capital Kuala Lumpur ahead of a large anti-government rally.
Police used tear gas and imposed a massive lockdown of the city in an attempt to prevent the opposition-led protest going ahead.
Around 300 people who gathered at a railway station were ordered to leave.
Witnesses say those who resisted were put into police vans.
Outside a downtown bus train, about 8000 protesters fought back tear gas and water cannon attacks from police. A standoff is now underway.
Opposition leaders had promised that tens of thousands of people would turnout to demand electoral reforms.
The protests could force Prime Minister Najib Razak to delay elections and painful economic measures that analysts say are essential.