Squashed into a sliver of an Alphabet City bar, patrons clapped into each other’s chests and tapped their toes long after midnight. Movement in any direction was futile. Michael Katsobashvili, known as Misha, the unofficial impresario in a white corduroy jacket and scarf, looked around at his closest friends. “There’s an old Russian saying,” he said to describe the room: “In tight quarters, but with no quarrel.” The trombone finished with a triumphant blast. “Opa!” Mr. Katsobashvili shouted, and the assembled chimed in. It was a typical Tuesday gathering at Mona’s, an otherwise nondescript bar on Avenue B off East 14th Street that hosts a weekly after-hours hot-jazz night from 11 p.m. until after 3 a.m. The music is a blend of Dixieland, riverboat jazz, bebop and blues, a collage of Americana that has attracted an international following and fervent crowds for the last several years.

“I’d rather play here than a lot of places with a famous name,” said Dennis Lichtman, 34, the clarinetist and leader of Mona’s Hot Four, the house band that kicks off with a set before opening the floor to an impressive collection of jazz musicians, who gravitate to Mona’s after finishing other gigs all over the city. There is no cover, and there are no airs either, just a revolving collection of musicians cramming into the corner and jamming around the old upright piano as night turns into murky morning. “This is Brigadoon,” said Peter Marcovicci, 67, a Long Islander who rarely misses a Tuesday. “One night of the week, it’s full of the best jazz musicians in New York. The next night, it’s a dive bar from ‘On the Waterfront.’ ” Bill Morse, 74, arrived at his usual bar stool by 10 p.m. to secure his place. His longtime friend John Casey, a rubber-stamp seller in the East Village, sat next to him, grousing about publicity. “Because we’ll get the wannabes here for three weeks,” Mr. Casey said. “We won’t be able to get seats, and when three weeks is gone, they’ll be moved off to the next big thing.” Mr. Casey has been coming for 20 years, since back when the bar had live Irish music. The Irish-born bartender Aidan Grant was not a big fan. So seven years ago, he proposed replacing it with hot jazz, right about the time a renaissance was unfolding among young musicians. He recently added an after-hours bluegrass night on Mondays. It’s popular, though less jam-packed. Charlie Levinson, 73, from Lakewood, N.J., passes the pitcher for donations, while his wife sits and claps. “The people who come here to listen to this music, the musicians, they are all so nice,” Gale Levinson, 67, said. “They’re very different from the rock ’n’ roll kids.” These “kids” often come in tuxedos after engagements around the city, with guitars, banjos and trombones strapped to their backs. Many come after sitting in with Vince Giordano and the Nighthawks, who play at the Iguana in Midtown. Through word of mouth, some come from overseas to sit in, like Dana Helz and Shachar El Natan, both 21, of the Tel Aviv band Rosetta’s Tone. Ms. Helz was treated to a master class this night. Cécile McLorin Salvant, a Grammy-nominated jazz singer, hushed the crowd as she began “Lover Come Back to Me.” This was her first time singing at Mona’s, and she nailed it. “It was a fun time, really cool to play through the older standards, especially in this setup, and with this out-of-tune piano,” Ms. Salvant said. “It’s very spontaneous; it’s like a party feel.” When she left, horn players crowded behind the bar, including Bria Skonberg, 30, a Canadian trumpeter. Ms. Skonberg and Mr. Katsobashvili are putting on the New York Hot Jazz Festival, which will feature 16 top bands. The event is at the Players Club in Gramercy on May 18. The after-party, of course, will be at Mona’s.