Malaysia's capital Kuala Lumpur was locked in early Saturday as heavy riot-police presence warned of clashes at a rally calling for electoral transparency, dpa reported
All major roads into the city were blocked from midnight and three bus depots were closed as part of the government's efforts to stop people from attending.
Organizers said they planned to hold the demonstration at the city's Merdeka Stadium in defiance of an official ban and police warnings of mass arrests.
Discussions with the authorities over an agreed venue broke down earlier in the week.
Officials accused organizers of threatening national security and attempting to overthrow the government, but the protest leaders insisted that they were only pushing for electoral changes and "a better democracy."
Activists are demanding the electoral registration be cleaned up and for greater transparency in election laws, as well as an end to "corruption and dirty politics."
The opposition-backed protest was expected to rally hundreds of thousands of supporters, but organizers said Saturday's turnout could be markedly less due to the "harassment and systematic arrests" of activists.
More than 250 activists had been arrested nationwide for promoting the rally.
Earlier in the week authorities held an exercise in which soldiers held up banners reading "disperse or we will shoot," and the army was placed on standby to be deployed for crowd control.
Prime Minister Najib Razak has supported the strict actions of the authorities, even throwing his endorsement behind local martial arts groups, including one that vowed to "wage war" against the activists.
Protest leaders said they had received death threats.
"The authorities have put obstacle after obstacle where they only needed to provide sincere cooperation to win the trust and confidence of the people," they said in a statement said Friday.
"(But) we have witnessed nothing but the utmost bravery and commitment to peace and justice demonstrated by ordinary Malaysians from every walk of life."