Mayor Thomas Menino last week rejected a controversial plan to redraw City Council boundaries, pointing to District 4’s high concentration of voters of color as a factor.

“I am returning, with disapproval, your ordinance even though I recognize your efforts,” he wrote in a Sept. 6 letter to the council, which is tasked with redrawing the boundaries of council districts every ten years in order to account for population shifts revealed by US Census figures.

“My central objection is my concern that the plan concentrates our many citizens of color into too few districts, and in doing so may limit their equal opportunity to elect candidates of their choice. The concentration of black voting age population and non-white voting age population in District 4 is of particular concern. Under the map passed, almost 70 percent of the district’s voting age population is Black and almost 95 percent is Non-White. In a city where diversity is found broadly, I ask that you endeavor to avoid over-concentration of minority voters.”

District 4 is represented by City Councillor Charles Yancey, and includes parts of Dorchester and Mattapan.

A coalition of voting rights groups and civic organizations had threatened to launch a lawsuit if the mayor signed the proposed map. The Boston NAACP, which is part of the coalition, said it will hold a community forum on redistricting on Sept. 18 at Hibernian Hall in Roxbury, starting at 6 p.m.

Cheryl Crawford, director of MassVote, another group in the coalition, urged for the chairman of the council’s redistricting committee, District 2 Councillor Bill Linehan, to attend the forum. “Our goal is simple—to unpack District 4 and to not crack District 2 – and now we just need to figure out how to get there with them,” she said in a statement.

The council, which voted 7-6 in favor of the proposal that Menino ended up vetoing, has a November deadline to come up with a new map, since any person running for a district council seat must be living in the district a year before the election. Members, after spending 16 months hashing out proposals, are eyeing Oct. 21 or Oct. 22 as a deadline to get a new proposal to Menino’s desk.

“It’s going to be hard,” City Council President Stephen Murphy said Tuesday, calling for more participation in the process from fellow councillors.

Those who said “no” to the Linehan map expressed relief over the mayor’s veto. “I am grateful Mayor Menino vetoed the redistricting map,” City Councillor At-Large Felix Arroyo, one of the six who voted against it, said in a statement. “I voted against the map because I believed we could do better. This is an opportunity to pass a map that best reflects our City and ensures everyone can have a voice in our government.”

City Councillor At-Large Ayanna Pressley, another opponent of the map who, like Arroyo, was down in Charlotte for the Democratic National Convention, took to Twitter, the social networking site, hours after the veto was announced. “I am glad to hear that Mayor Menino vetoed the redistricting plan,” Pressley said. “Back to work.”

District 7 City Councillor Tito Jackson said councillors should take another look at his map, which moves around fewer precincts than the Linehan map, which relocates the Polish Triangle (Ward 7’s Precincts 8 and 9) into District 3 and hands over Lower Mills precincts (Ward 17’s Precincts 12 and 14) to District 4. District 3, represented by Frank Baker, who voted for the proposal, also gains Ward 7, Precinct 7, which includes Carson Beach, and Ward 7, Precinct 10 and Ward 13, Precinct 5.

Yancey praised Menino’s veto, but remained adamant about placing Mattapan all in one district, which has put him at odds with activists and his colleagues. “Mattapan has to be reunited,” he said.