(Reuters) - Activists have filed a federal lawsuit asking for a new primary election in Baltimore, alleging irregularities during the tight April 26 mayoral vote.

The lawsuit was filed by Voters Organized for the Integrity of City Elections, or VOICE, late on Wednesday against the city and state election boards. It asks the U.S. District Court to declare the primary election results invalid, order a new election and appoint federal observers to oversee the procedure.

State Senator Catherine Pugh narrowly won the Democratic mayoral primary, defeating former Mayor Sheila Dixon. Pugh had backed law enforcement reform as Baltimore recovers from unrest in April 2015 sparked by a black man’s death from an injury suffered while in police custody.

An investigation by the state election board found that Pugh won the nomination by 2,400 votes. About 1,700 ballots cast in the primary were handled improperly, the probe found.

Dixon said on Wednesday that she would not file a lawsuit challenging the election outcome. Spokesmen for the two election boards did not immediately respond to requests for comment.

The lawsuit alleged improper handling of the ballots. It also alleged that voters were hampered by polling places that opened late and by a letter mistakenly sent by election officials to ex-offenders saying they might not be able to vote because of their convictions.