The opposition Cambodia National Rescue Party organized protests against the ruling Cambodian People's Party, which won the majority of the parliamentary seats in a July 28 election.
London said roadblocks were set up near the Cambodian capital, Phnom Penh, during a "mass" weekend protest. It said some clashes occurred between protesters and police. A military spokesman told China's official Xinhua News Agency authorities fired tear gas and water cannons on demonstrators Sunday.
The FCO said protests are set to continue through Tuesday. It said Monday it was advising "against all travel to parts of the country," especially the western border with Thailand, because of the potential for further unrest.
Opposition leaders said they'd boycott the Sept. 23 opening session of Parliament unless authorities examined allegations of election irregularities.