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PHNOM PENH (Reuters) - Cambodian opposition members lit candles and held prayers in Phnom Penh on Friday to mark the 21st anniversary of a deadly grenade attack - a rare public gathering since a government crackdown started last year.

The opposition Cambodia National Rescue Party (CNRP) was banned in November after the arrest of its leader Kem Sokha on treason charges, which he says are political motivated ahead of a national elections in July.

Opponents of Prime Minister Hun Sen accuse him of arresting Kem Sokha and cracking down on independent media and other critics in the face of what could have been his toughest electoral challenge in more than 30 years in power.

Hun Sen has ruled Cambodia for 33 years and is now widely expected to win the election on July 29. Western countries have said the election cannot be fair without an opposition.

Around 40 opposition members, joined by Buddhist monks, gathered at a memorial in the capital to mark an attack on opposition supporters that killed at least 16 people and left hundreds wounded in 1997.

Last year’s anniversary ceremony drew hundreds of people.

This year’s ceremony was held under the auspices of the Candlelight Party, linked to former opposition leader Sam Rainsy, who was himself wounded in the 1997 attack and who fled Cambodia in 2015 to escape a defamation conviction.