A court in Cambodia has charged two men with the murder of union leader Chea Vichea nine days ago.
Chea Vichea's supporters are convinced the killing was political
"I am the shooter," one of the suspects, Born Samnang, told the Phnom Penh municipal court.
His co-defendant, however, denied any involvement in the murder of the leader who was associated with the opposition Sam Rainsy Party.
Vichea's death was the latest suspected political killing apparently targeting government critics in Cambodia.
Chea Vichea was president of Cambodia's Free Trade Union of Workers (FTUWKC), one of the country's most active unions, with about 30,000 members.
About 10,000 attended his funeral.
Cambodia came under intense pressure to investigate the murder fully.
The UN's acting High Commissioner for Human Rights, Bertrand Ramcharan, said those who ordered the killing - as well as those who carried it out - should be found and prosecuted.
He said democracy in Cambodia was being hindered by the fact people felt they could act without worrying about the consequences.
Born Samsang said he and Sok Sam Oeun had been offered $5,000 to carry out the murder - but he had only received $750.
"I used a gun to shoot him and Oeun was my driver," he said, according to the French news agency AFP.
Sok Sam Oeun maintained his innocence and said he did not know Born Samnang.
Thousands took part in the funeral
"It is an injustice for me, I did not conduct that crime," Sok Sam Oeun told the court.
Sam Rainsy, leader of the opposition party which bears his name, has said that Chea Vichea "was a target of the current regime", because of his stance against corruption and human rights abuses.
The union has also said that the killing was politically motivated.
Cambodia is still without a proper government after the ruling Cambodian People's Party (CPP) failed to win an overall majority in last year's elections.
The opposition parties have warned that the murder of the labour leader is likely to cast a shadow over negotiations currently under way to form a government.
At least three other members of the opposition have been killed in recent weeks.