Sam Rainsy, who was greeted by huge crowds yesterday after his return from self-imposed exile in France, is seen as the main challenger to strongman Prime Minister Hun Sen.
But he has been removed from the electoral register and is unable to run as a candidate in the July 28 polls unless parliament amends the law.
In an interview with Radio Free Asia, Rainsy warned that demonstrations might break out if he is not allowed to stand.
"Then we will demand a real election to allow Cambodians to decide their true destiny," he added.
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US lawmakers have called for the United States to cut off aid to Cambodia unless the vote is free.
Rainsy, a French-educated former banker, fled in 2009 to avoid charges he contends were politically motivated.
He was convicted in his absence of charges including inciting racial discrimination and spreading disinformation and had faced 11 years in jail but received a royal pardon earlier this month.
Although he is not a candidate, he has hit the campaign trail to spearhead his party's efforts to end Hun Sen's nearly three decades in power.
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.
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.
The opposition also accused people in power of being behind an attack by an unknown gunman who fired a bullet at the party's headquarters in the early hours today.