By Guy Delauney
BBC News, Phnom Penh
Hun Sen has been Cambodia's prime minister since 1985
Political parties in Cambodia have begun their campaigns in the run-up to next month's general election.
Thousands of supporters of the main parties have been holding rallies around the country and there have been processions in the capital, Phnom Penh.
There are high hopes that Cambodia's fourth democratic election will be a peaceful one.
But there have been concerns about legal moves against the main opposition leader, Sam Rainsy.
The mobile sound system operators are out in force.
Dozens of small, flat-bed trucks are criss-crossing Phnom Penh, blasting out music, political slogans - anything to get the attention of potential voters
Party members cram into the back of each vehicle - the ruling Cambodian People's Party (CPP) supporters all in white, their coalition partners Funcinpec in yellow.
Sam Rainsy is the main challenger to Hun Sen
Supporters of the main opposition Sam Rainsy Party wave placards bearing the bespectacled face of their leader. Police hold up the traffic to let the processions through.
It is a rare chance for parties to rally their supporters. The authorities usually place severe restrictions on large gatherings and marches.
But now there will be a month of campaigning for the elections, which take place on 27 July.
Martin Callanan is the chief observer from the European Union's election monitoring mission.
"There are obviously a few difficulties in terms of campaigning for some people during the election," he says.
"But the atmosphere generally seems to be so far, fingers crossed, not as tense. And there's certainly not as much violence as there has been in previous elections."
The European Union is sending more than 100 observers. They will be joined by almost 14,000 monitors from local organisations.