Thousands of supporters were accompanying killed Cambodian political analyst Kem Ley's body to temple, where the first stages of his Buddhist funeral were to be observed Sunday evening, after he was gunned down in a Phnom Penh street earlier.
The crowd included several dozen monks, who marched alongside supporters holding photos of the slain activist.
A frequent critic of Prime Minister Hun Sen, Kem Ley was shot three times Sunday morning by an assailant with a handgun at a petrol station, an unnamed police officer was quoted saying in the Phnom Penh Post.
The incident occurred around 9 am (0200 GMT), and within 90 minutes police officers said they had arrested a suspect, the report said.
"The suspect said the motive of the killing was about [Kem Ley] owing money. But we don't believe him yet," said National Police spokesman Kirth Chantharith, according to the Post.
"We are still questioning him," the spokesman added.
Kem Ley founded an advocacy group in 2014 to promote local political groups throughout the country, the Cambodia Daily said.
He regularly appeared on television and radio to discuss Cambodian politics.
The killing comes days after a damning Global Witness report accused the government of untold corruption.
"Global Witness strongly condemns the murder of Kim Ley," the NGO said in a statement, adding that Ley was not a source in their report. "We are calling for a thorough and independent investigation into Ley's murder."
The mood on the streets of the capital was bleak. "We know he had a problem with Hun Sen," said one tuk tuk driver whose friends nodded in agreement.
Sunday's protesters, who chanted for justice, passed peacefully.