A gunman fired a shot into one of the main offices of Cambodia's opposition party, a day after the exiled leader of the opposition returned home to an exuberant welcome by supporters ahead of this month's general elections.

The office in the capital Phnom Penh was closed for the night and nobody was injured by the attack early on Saturday morning. But it was immediately denounced as an effort to intimidate the opposition after the long-awaited return of its leader Sam Rainsy.

"This attack was orchestrated by those in power," said opposition party spokesman, Yim Sovann. He said security guards were sleeping when the shot shattered a window at about 3am local time.

They shot at our office to attack our spirit ahead of the election, but this attack doesn't scare us at all

Yim Sovann, Cambodia National Rescue Party spokesman

"They shot at our office to attack our spirit ahead of the election, but this attack doesn't scare us at all," he said.

Police were sent to investigate, said Phnom Penh's police chief, Lt Gen Chhun Sovann.

Huge, exuberant crowds turned out on Friday to greet Rainsy when he returned home to spearhead his party's election campaign against well-entrenched Prime Minister Hun Sen.

The French-educated leader of the Cambodia National Rescue Party had been in exile since 2009 to avoid serving 11 years in prison on charges many consider politically motivated.

Rainsy, 64, received a royal pardon last week at the request of Hun Sen, his bitter rival whose ruling party is almost certain to maintain its ironclad grip on power in the July 28 general election.

Hun Sen has ruled for 28 years, and his party has 90 of the 123 seats in the National Assembly. The 60-year-old prime minister recently said that he intends to stay in office until he is 74 - cutting back from an earlier vow to stay in control until he is 90.

Rainsy's pardon came after the US and others had said his exclusion from the campaign would call into question the polls' legitimacy. Because he was absent during the registration periods, he will be unable to run as a candidate, or even vote, although his lawyers have said they were seeking a way to allow his participation.

Source: Agencies