By Prak Chan Thul

PHNOM PENH (Reuters) - Thousands of people attended a funeral procession in Cambodia's capital on Sunday for a prominent government critic, Kem Ley, whose killing has raised fears about political violence in the run-up to elections.

Kem Ley was shot dead in a shop in Phnom Penh on July 10. Police have arrested a suspect and say they believe the motive for the murder was a debt.

Government critics in a country with a long record of violent politics believe Kem Ley was killed because of his work as an activist and his political analysis though they say there is no evidence for that.

"I am here to accompany him for the last time," said Chhouk Da, 28, a volunteer helping to direct traffic as the procession made its way with Kem Lay's body in a refrigerated glass casket on a truck, flanked by Buddhist monks.

"He was just helping the nation," said Chhouk Da who was wearing a T-shirt with the words "Wipe your tears, continue your journey".

The killing comes at a time of rising political tension between Prime Minister Hun Sen and an opposition hoping to challenge his 30-year grip on power in local elections in 2017 and a general election in 2018.

Members of the opposition and activists have been jailed on charges they say were trumped up by the government as part of a crackdown to mute critics.

Mourners wearing white shirts held small flags, lotus flowers and pictures of Kem Ley, 46, as the procession including motorbikes and cars moved out of the capital to Kem Ley's home province of Takeo, about 70 km (40 miles) to the south.

Anti-riot police were on guard outside government buildings along the route.

Human rights group Licadho said the procession was at least 8 km (5 miles) long with thousands of people also lining the route.

A police video posted online showed the arrested suspect, Chuop Somlap, 38, whose name means "meet to kill", saying he killed the political commentator over a $3,000 debt.

The United States has called for a credible investigation. The European Union and United Nations have expressed concern about political tension.

Human Rights workers say the killing is bound to have a chilling effect on political activism.

"The assassination of Kem Ley - a calamity in itself - sadly reaffirms the primacy of violence in the politics of Cambodia," John Coughlan, a researcher at Amnesty International, said in a statement.

(Reporting by Prak Chan Thul; Editing by Amy Sawitta Lefevre and Robert Birsel)