Campaigning begins today and will continue for a week. The 136 candidates are running for 68 seats in Iran's 290-seat parliament, for constituencies in which no candidate achieved a clear majority during the general election.
In February's election, a bloc of reformists and moderate allies of President Hassan Rouhani won a majority in parliament and in a clerical body known as the Assembly of Experts, which is charged with choosing the country's next supreme leader.
The results were seen as a major setback for hard-liners opposed to the nuclear deal reached with world powers last year.