Cambodia's newly returned opposition leader Sunday applied to be reinstated as an electoral candidate in order to stand against strongman Prime Minister Hun Sen in polls this month.

Sam Rainsy, who was greeted by huge crowds on Friday after his return from self-imposed exile in France, was removed from the electoral register late last year and is unable to run as a candidate in the July 28 polls unless parliament amends the law.

He is seen as the main challenger to Hun Sen.

In a letter to the president of the National Election Committee (NEC), Rainsy asked whether it was possible to find a way "to reinstate my name in the voter list of NEC and the official list of MP candidates".

He is asking to contest in southern Kandal province where Hun Sen is also standing as a candidate, according to the letter.

Rainsy, who returned after receiving a surprise royal pardon this month, has already hit the campaign trail to spearhead his party's efforts to end Hun Sen's nearly three decades in power.

In an interview with Radio Free Asia Friday the French-educated former banker warned that demonstrations might break out if he is not allowed to stand.

"If I can't participate, after the elections all the Cambodian people will protest and the whole international community will condemn the result and regard this as a sham election," Rainsy was quoted as saying.

Neither Rainsy nor officials from his Cambodia National Rescue Party (CNRP) could be reached for comment.

Tep Nytha, NEC's secretary general, said he was unable to say immediately whether or not Rainsy would be reinstated.

"NEC will meet and decide about the request in accordance with its power and electoral law," he told AFP, adding that as the law stands, reinstating him would be "impossible".

US lawmakers have called for the United States to cut off aid to Cambodia unless the vote is free, while the UN's special rapporteur on human rights in Cambodia, Surya Subedi, last week urged Cambodia to let Rainsy play a "full part" in politics.

Rainsy fled in 2009 to avoid convictions he contends were politically motivated.

He was found guilty in his absence of charges including inciting racial discrimination and spreading disinformation and had faced 11 years in jail until he was pardoned.

Despite exile, he has remained active in Cambodian politics and recently joined with former rivals to create the CNRP in an effort to finally unseat Hun Sen.

Hun Sen is one of Southeast Asia's longest-serving leaders. His Cambodian People's Party (CPP) won the last two polls by a landslide amid allegations of fraud and election irregularities.

His government is regularly accused of suppressing political freedoms and muzzling activists. In May Hun Sen said he would try to stay in power for another decade.