Baltimore elections officials mistakenly sent letters to 34 ex-offenders saying they might not be able to vote because of their convictions.
Early voting began Thursday throughout Maryland. Under a state law that took effect in March, people with felony convictions can register to vote as soon as they are released from prison.
Previously, felons could not vote until they completed probation or parole.
But shortly after the law took effect, 34 people were "inadvertently processed based on an old procedure," Armstead B.C. Jones Sr., the city elections director, said in a statement Thursday.
He said elections officials are contacting those people to inform them of the error.
"All of these voters are all eligible to cast a ballot in Maryland's 2016 Presidential Primary election," Jones said in the statement.
John Comer, co-director of Communities United, said he had heard the issue was being resolved but was nonetheless concerned about the message the letters sent.
"We're talking about a population that feels disenfranchised already," Comer said. "Letters go out of this nature, and it can send a message that we don't want you voting, we don't want you re-enfranchised."
Activists have estimated that the new law will make about 20,000 people in Baltimore — and 40,000 people statewide — eligible to vote. As of 5 p.m., more than 4,000 people had cast ballots in Baltimore on the first day of early voting. State officials reported more than 30,000 early voters — twice as many as in the last presidential race.
Also Thursday, Del. Cory McCray — a Baltimore Democrat who sponsored the bill to restore felons' voting rights — joined with several groups that called upon elections officials to "remove barriers to voting" in Baltimore.
McCray said he was concerned when, a few weeks ago, he was filling out an application for an absentee ballot and saw a question about being convicted of a felony.
Kimberly Haven, founder of the Catalyst Collaborative for Innovative Social Change, said activists will be looking out for problems. She had not heard of anyone not being allowed to vote because of their conviction, she said.
"If something happens, we will jump on it," Haven said.
The new law makes the message to voters simple, she said: "As long as you're not in a jail cell, you can vote."
Baltimore Sun reporter Luke Broadwater contributed to this article.