Sri Lanka's former president says he has learned from his election defeat and promises a new approach to relations between ethnic communities as he runs for prime minister in next month's elections.

Mahinda Rajapaksa, who was defeated in January presidential polls, launched his political comeback campaign in the northwestern city of Anuradhapura on Friday.

In a speech, Rajapaksa denied allegations of corruption and misrule and said he was a victim of false propaganda.

Rajapaksa was credited with defeating ethnic minority Tamil separatists in a civil war, but accused of enflaming ethnic divisions while sustaining his ethnic Sinhalese voting bloc.

If Rajapaksa's party secures a majority in Parliament he could become prime minister, a step away from president.