Reuters -- President Hassan Rouhani has an unassailable lead in Iran's presidential election, an Iranian official source told Reuters on Saturday, citing an early unofficial tally, and is on course to defeat his rival Ebrahim Raisi.

"It's over, Rouhani is the winner," the source said on condition of anonymity.

Rouhani had won 21.6 million votes, compared to 14 million for Raisi, with 37 million votes counted, the source added. There were about four million more votes still to be tallied, the source said.

More than 40 million ballots were cast, the interior ministry said, indicating a turnout of about 70 percent in Friday's vote, roughly similar to the showing in 2013 elections when Rouhani swept into office in a landslide victory.

State television said the official early results would be announced soon.

Rouhani's chief of staff Hamid Aboutalebi tweeted that Rouhani had won 60 percent of the vote. He cited no evidence.

In the last election, Rouhani won more than three times as many votes as his closest challenger. But this time the outcome might be much closer, as other rivals have backed out and thrown their support behind Raisi.