Drake, seen at the Juno Awards in April, told fans he was disappointed by the cancellation of his free concert. (Ryan Remiorz/Canadian Press)Police cancelled Canadian rapper Drake's free outdoor concert in New York City on Tuesday night after nearly twice the expected number of fans showed up and grew into an unruly mob.

The evening concert scheduled the same day his much anticipated album Thank Me Later debuted began at about 6 p.m. local time with a performance by opening act DJ Ninjasonik at Manhattan's popular South Street Seaport.

The show's headliners were pop group Hanson, the Oklahoma trio of brothers, and Drake, the Toronto-born rising hip hop star.

Organizers had anticipated about 10,000 people would turn up, but as the evening wore on, the crowd grew to an estimated 20,000, according to police.

Fans began climbing atop roofs and pushing towards the stage. A few scuffles broke out amid the raucous crowd, while some rowdy individuals began tossing bottles, flower pots and even chairs from upper sections of the Seaport building.

According to New York police, several 911 calls were made about the increasingly unruly situation and, at approximately 7 p.m., organizers appealed to authorities to help disperse the crowd.

Officers ultimately arrested two people on minor charges and provided aid to several others with minor injuries.

Drake later issued a message to fans via his Twitter account that expressed his disappointment at the axed concert.

"To all the devoted fans that came out, I wish you could have seen what I had planned! Until next time," he tweeted.

The concert was organized by indie culture title Paper magazine as part of its Sounds Like Paper series.

Magazine staff apologized for the cancelled show in a posting on its website, saying: "Wow, the crowd was much larger than we anticipated, and unfortunately the show was canceled. But we will make it up to you guys!"

With files from The Associated Press