As the celebration at Fenway carried into Thursday morning, hundreds of people assembled at the finish line of the Boston Marathon. Traffic stopped. And some knelt down to touch, or kiss, the blue and yellow stripe on Boylston Street.

It was a moment that defined what this World Series championship meant: the crowning achievement of a team’s rise from the ashes of last year’s lost season, and a city’s rise from its darkest hours last April.

“I was in the city [on April 15] and this just feels so different to have a good feeling. It’s incredible,” said John Kmetz, 25, of Topsfield, walking by the finish line. “I just wanted to be here to feel it.”

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What started as a joyful celebration, however, turned rowdy as crowds leaving the Fenway area converged on Boylston. Nearly 1,000 people blocked traffic and a handful began climbing on cars. About two dozen revelers tried to flip a car parked in front of the Talbots store as police arrived and cleared the crowd.

Boston police said another car was flipped near Charlesgate Street.

“We are dealing with crowd control issues,” spokeswoman Cheryl Fiandaca said in an e-mail. As of about 2 a.m. on Thursday, 12 people had been arrested, police said, and the crowds were under control.

Red Sox Nation had come to witness a victory Wednesday that made Boston the first three-time World Series champions in the 21st century, the first time the Sox clinched at home since 1918 the date once synonymous with playoff futility and a championship drought that lasted 86 years.

Matt Harrington, of Simsbury, Conn., watched the Sox break the curse in 2004 and also saw the bombs go off in April. He said his emotions were mixed.

“I was here in ’04. It was crazy. But this was more,” he said, after smacking the finish line with his fist.

Earlier, longtime fans got the chance to witness the history they had waited for at 11:13 p.m. as the last Cardinal struck out.

The city erupted in cheers of “Boston Strong, Boston Strong,” revelers on Commonwealth Avenue in Kenmore Square screamed, threw strings of beads, and sprayed carbonated liquids in the air as a bagpipe buzzed in the background.

Some jumped into a flatbed trailer attached to the back of a Boston police pickup truck, leaping up and down in unison as the trailer bounced. Minutes later, police forced them away from Kenmore Square, moving barricades to steadily push people from any access point to Fenway Park.

Hundreds of revelers spilled out of Allston bars, unleashing their “let’s go Red Sox chants “ along Brighton Avenue, which was dotted with police and cruisers with lights flashing. Prevented from assembling on Copley Square, fans filed past each other, exchanging high-fives.

They had packed restaurants along the Marathon finish line. They had jammed Fenway to the rafters so that standing-room areas on the main concourse were stacked three-to-four people deep, leaving fans straining not just to see the field, but even to view the TV feed on the monitors around the stands.

The last time Spencer Koffman, 57, was in Fenway Park, Yaz was playing left field. That was 1974 and Koffman was living in Sharon. He moved to Oakland and, decades ago, he made a promise.

“I told myself I am coming back to Fenway when the Sox are going to win a World Series here,” he said.

The crowd at Fenway had erupted as one when the Sox’s Shane Victorino swatted a three-run double off the Wall and when shortstop Stephen Drew broke a playoff-long slump with a home run. Strangers hugged and slapped high-fives. The cheers echoed across Boston.

At Fernandez Barber Shop in Jamaica Plain, where Sox designated hitter and team leader David Ortiz sometimes gets his hair cut, workers kept the place open so that friends and family could watch.

“Es una derrota!” said Luis Sosa, 37, who lives in the neighborhood. “It’s a rout!”

As the game progressed, the outcome seemed inevitable.

“Just bring in the Duck Boats already.” said a diner at UBurger, one of the unassuming establishments that served as overflow rooms of sorts for Fenway’s most popular sports bars, packed solid by the start of the game. Meanwhile, early and unofficial indications were that a victory parade would be held on Saturday.

Inside Qdoba Mexican Grill in Kenmore Square, fans crowded the entrance to catch a glimpse of the game on the single TV in the corner, straining to hear the commentary over the pop tunes streaming from the speaker system.

Then there were the luckier ones. Up in the Green Monster seats, C.J. D’Arezzo, of Rhode Island, sat with his daughter Arianna, 10. They had bought the seats from a reseller earlier Wednesday for $1,900 each.

“Worth every penny,” he said.

Some came from afar.

Kimi Puntillo, a St. John’s University professor who teaches sports writing at the New York school, hustled to Game 6 on Wednesday after buying a box seat on StubHub after class.

“I thought, there might not be another World Series again in my lifetime,” said Puntillo, a former Tufts student. “I got here just in time for the first pitch.”

Music of all kinds became the songs of Fenway: “I’m Shipping up to Boston” performed on the diamond before the game by the Dropkick Murphys; Bob Marley’s “Three Little Birds” sung by fans in the stands. “Take Me Out to the Ball Game” played by a trombonist on the Massachusetts Turnpike overpass. And of course, a punch-drunk, screaming rendition of “Sweet Caroline.”

The atmosphere outside Fenway Park before the game was circus-like, as ticketholders mixed with revelers simply hoping to soak in some of the excitement. Men with booming voices hawked programs, pennants, and fake mustaches — “Gotta love that dirty water, folks!” yelled one, to no one in particular – while a bagpipe player, a Hare Krishna troupe, and an evangelical Christian minister on a megaphone added to the cacophony.

Greg Wolfe, 57, of Easton, and Paul Charpentier, 61, of Cranston, R.I., were lucky enough to have first-row seats behind home plate, where they were settled an hour before the game.

Wolfe wore the Oliver Ames High School baseball cap that his son, Brendan, had worn for the school’s team before he passed away three years ago at 17.

“I sat in these seats with him 10 years ago for an early-season game,” Wolfe said. “I feel blessed to be here. There are some moments that aren’t so good, but then there are moments like these.”