Updated 21 May 2013,
Listen to audio at Radio Australia
The demonstrators submitted a petition at the EU delegation asking for an intervention and marched to the United Nations rights office where leaders briefly met with visiting UN envoy Surya Subedi.
The march was organised by the Cambodian National Rescue Party, which is a merger of the former Sam Rainsy Party and the Human Rights Party. It's the main opposition force against the powerful and well-entrenched CPP.
The Cambodian People's Party has ruled since the early 90's and Prime Minister Hun Sen has said he plans on staying in power for another 15 years.
Over the years, the political opposition has been met with grenade attacks, targetted assassinations and harrassment of its members and one of the key opposition voices, Sam Rainsy, is currently living in self-imposed exile.
He's currently in talks with the Cambodian Government to allow him back into the country.
Presenter: Liam Cochrane
Speaker: Sam Rainsy, President of the Cambodian National Rescue Party
RAINSY: Those talks conducted through the United Nations, a number of friendly governments.
COCHRANE: Can you give us a little bit more information about when they took place?
RAINSY: For instance the UN Special Envoy, Surya Subedi, is currently in Cambodia and he is going to meet Cambodian top officials to discuss among other things the holding of elections next July.
COCHRANE: Is he expected to meet Prime Minister Hun Sen?
RAINSY: I think so.
COCHRANE: In the past that has been difficult and he has not been able to get an audience with the Prime Minister. What makes you think he will this time round?
RAINSY: Because Cambodia cannot afford to be isolated and cannot afford to reject the UN recommendations for the forthcoming elections, otherwise any government arising from illegitimate elections would be considered illegitimate also.