Ahead of the final rallies later on Saturday, opposition candidate Sarath Fonseka predicted that the ruling party, led by President Mahinda Rajapakse, would use violence to intimidate his supporters in the Tuesday vote.
"They want to create violence and discourage people from voting.
A lower turnout will help rigging," he told reporters, adding that he expected fake voters and forged ballot papers to also be used.
In the run-up to the poll, police say at least four people have been killed and hundreds wounded in clashes between supporters.
The house of an influential opposition figure was bombed on Friday.
Fonseka said he and his allies would rely on the protection of the security forces, most of whom he said would vote for him except for a few senior figures who are "trying to please the government.
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"We can't counter violence with violence. We have to abide by the law," he said.
The ruling party has denied allegations that it plans to rig the voting and independent election com