Dozens of security personnel armed with shields and batons flooded into the area in central Phnom Penh, causing several hundred protesters to flee, according to an AFP photographer at the scene. There did not immediately appear to be serious clashes as a result of the move.
It comes a day after a crackdown on textile workers that left at least three dead, and which rights campaigners condemned as the country's worst state violence against civilians in more than a decade.
Phnom Penh governor Pa Socheatvong said in a statement that the opposition Cambodia National Rescue Party (CNRP) would not be allowed to hold demos or marches "until the security situation and public order is restored to normal".
The opposition party, which has boycotted parliament since a disputed July election that returned Hun Sen to power, had planned a major three-day protest starting from tomorrow.
"This is the act of communist dictatorship," opposition spokesman Yim Sovann told AFP.
Protesters have occupied Democracy Park since December as part of demonstrations against premier Hun Sen's government that swelled to an estimated 20,000 or more opposition supporters on the streets last Sunday.
The recent violence saw striking workers armed with sticks, rocks and Molotov cocktails clash with rifle-wielding police in the Veng Sreng factory district of Phnom Penh yesterday.
The UN's special rapporteur on human rights in Cambodia, Surya P Subedi, criticised the shootings, calling on the government to launch an investigation.
The Cambodian Centre for Human Rights, an independent activist group, has said at least 25 demonstrations were violently repressed in 2013 by security forces using guns, tear gas, water cannon and batons, leaving two people dead, one person paralysed and causing three women to suffer miscarriages.
