Opinion surveys two weeks ahead of presidential and legislative elections in the world's third-largest democracy show Indonesian President Joko Widodo and his ruling coalition maintaining a large lead over the rival camp.

Four nationwide surveys conducted in the second half of March show Widodo between 13 and 20 percentage points ahead of former special forces general Prabowo Subianto.

The race has tightened modestly since late last year, but pollsters say Widodo, who has had a commanding lead for months, is headed for victory in the April 17 election.

Widodo, the first Indonesian president from outside the Jakarta elite, has campaigned on progress in upgrading inadequate infrastructure and reducing poverty.

Far behind in the polls, Prabowo's team has resorted to questioning the integrity of the upcoming vote.