Fans climb atop a roof at free concert featuring Drake and Hanson

*A free NYC concert by rapper Drake and pop band Hanson had to be scrapped Tuesday after twice as many people as anticipated showed up and started tripping. [See video below.]

The show at Pier 17 on Manhattan’s South Street Seaport started at about 6 p.m. with an opening act. Hanson, of “Mmmbop” fame, and headliner Drake, whose album “Thank Me Later” dropped Tuesday, were to appear on stage soon after.

Organizers were anticipating about 10,000 people but nearly 20,000 showed up, police told the Associated Press. Folks began climbing on rooftops and some were tossing bottles, as concertgoers smashed together toward the stage, police said.

Witness Tamika Johnson told the New York Daily News: “People in the front started leaving because they were getting crushed.”

Show organizers sought assistance from NYPD in dispersing the crowds at around 7 p.m., police spokesman Paul Browne said. Two people were arrested on minor charges; six suffered minor injuries, police said.

The concert was part of the “Sounds Like Paper” series presented by Paper magazine, an independent publication focusing on arts, culture and music. The magazine apologized on its website: “Wow, the crowd was much larger than we anticipated, and unfortunately the show was canceled. But we will make it up to you guys!”

A rep for Drake, whose album features Kanye West and Alicia Keys and debuted to positive reviews, told the Daily News the rapper was disappointed the NYPD shooed away crowds before he could take the stage.

Below, clips of the packed crowd before things got out of hand.