DUBAI, May 20 (Reuters) - Iranian President Hassan Rouhani leads in Iran's presidential election, an Iranian source told Reuters, citing an early unofficial tally.

"It's over, Rouhani is the winner," the source said.

Rouhani had won 21.6 million votes, compared to 14 million for his hardline conservative challenger Ebrahim Raisi, with 37 million votes counted, the source added. There were about four million more votes to be tallied, the source said. (Reporting by Parisa Hafezi, Editing by illiam Maclean)