NEW YORK — A free concert by up-and-coming Canadian rapper Drake and pop band Hanson was cancelled Tuesday after twice as many people as anticipated showed up and many became unruly, police said.

The show started at about 6 p.m. with an opening act at Pier 17 on Manhattan's South Street Seaport, a popular summer locale for concerts. Hanson, the Tulsa, Okla., brothers behind the 1997 hit "Mmmbop," and headliner Drake, whose album "Thank Me Later" debuted Tuesday, were to appear on stage soon after.

Organizers were anticipating about 10,0000 people but nearly 20,000 showed up, police said. Many people climbed roofs and started throwing bottles, as concertgoers smashed together toward the stage, police said.

Witness Tamika Johnson told the Daily News newspaper: "People in the front started leaving because they were getting crushed."

The concert organizers asked the New York Police Department for help dispersing the crowds 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 magazine 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!"

The magazine posted several updates on Twitter during the night: "It's a total madhouse -- in a good way" and "get off the roof!"

Telephone calls by The Associated Press to representatives for Drake and Hanson weren't immediately returned Tuesday night.

A spokesman 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 dispersed the crowds before he could perform.

Hanson's "Mmmbop," from the trio of brothers' album "Middle of Nowhere," was one of the biggest debut singles of all time.