PORTLAND — One of today’s hottest folk rock bands has agreed to play on the Eastern Promenade this summer in what’s expected to be Portland’s largest outdoor concert – other than July 4th celebrations – in years.

The State Theatre, which is promoting the event scheduled Aug. 4, anticipates that more than 12,000 people will attend the “Gentlemen of the Road” concert featuring the British band Mumford & Sons.

Organizers hope that the daylong event, on a Saturday, will spill over into the rest of the city by nightfall. For the “after concert party,” smaller, indoor venues would host the local musicians who will be opening acts for Mumford & Sons.

Portland’s City Council approved the concert proposal Monday night by an 8-0 vote. On the advice of its attorney, the council stipulated that the State Theatre agree to a contract detailing conditions that must be met to protect the public’s safety.

Lauren Wayne, general manager of the State Theatre and State Theatre Presents, said she learned two weeks ago that Portland had been chosen as one of only four cities in the United States, and eight worldwide, to host the Mumford & Sons 2012 concert tour.

“It speaks volumes for our city,” Wayne told the council. “I can’t wait to show off my city to the rest of the world.”

No information on ticket prices, sale dates or the backup bands has been announced.

Mumford & Sons formed in 2007 and quickly won critical raves and a loyal following.

The band’s debut album, “Sigh No More,” was released in 2010 and eventually reached No. 2 on the Billboard 200 album chart. It was first on the Billboard Folk Albums chart.

The band was nominated for Grammy awards in 2011 in the best new artist and best rock song (“Little Lion Man”) categories. This year, the band’s single “The Cave” was nominated for Grammy Awards for best record of the year, song of the year, best rock performance and best rock song.

In the proposal to the city, Wayne said a portion of the Eastern Promenade park will be fenced off to accommodate people with tickets to the concert. Wayne said about two-thirds of the park’s hillside will be closed to the public. The main entrance to the concert will be across from Moody Street.

Gates will open at noon, with the concert ending by 9 p.m. Alcohol will be served at a beer garden.

For boaters, traffic will be allowed on Cutter Street until 2 p.m. on the day of the concert. Cutter Street will then be closed until 7 a.m. on Aug. 5.

Wayne said the playground, the tennis courts, the Eastern Prom Trail and the East End Beach will remain open.

Despite its scope and the additional traffic the concert will bring to Munjoy Hill, no one spoke against the plan at Monday’s council meeting.

Wayne indicated in her proposal that a portion of the concert proceeds will go to Friends of the Eastern Promenade, which is developing a restoration plan for Fort Allen Park on the prom.

Friends of the Eastern Promenade wrote a letter to the city supporting the State Theater’s concert plan.

“Portland’s premier green space, the Eastern Promenade, provides the ideal venue for an outdoor music event such as this. As we understand it, Mumford & Sons draws a mellow following of grassroots folk rock lovers who will come not only from throughout Maine but from all of New England,” wrote Diane Davison, president of Friends of the Eastern Promenade.

Ralph Carmona, who serves on the Munjoy Hill Neighborhood Association’s board of directors, said his group also supports the concert.

“We have concluded that this is a good deal,” Carmona said. “It’s an opportunity to showcase the city and the Eastern Promenade.”

Wayne estimates that the crowd could reach 12,500. The Cumberland County Civic Center, by comparison, has a concert capacity of about 8,000.

Portland has had outdoor concerts in the past, including at the Ocean Gateway Terminal and the Maine State Pier. None has had space for 12,000 spectators.

– Staff Writer Ray Routhier contributed to this report.

Staff Writer Dennis Hoey can be contacted at 791-6365 or at: [email protected]