I Am Legend opened with takings of $76.5m (�37.9m) in the US

The actor signed autographs for nearly an hour despite sub-zero temperatures.

He told fans: "I just wanted to say thank you very much - it's cold out here but you've made my heart feel warm. Thank you!"

Smith, 39, plays a military scientist who is the last surviving resident of New York City after a virus wipes out the population.

He told Radio One Newsbeat about how he researched the role.

"I found a couple of guys who had been former prisoners of war and people who were in solitary confinement, and they just walked me through the psychological deterioration of being confined," he said.

Smith added that he was pleased the film had already done so well at the US box office.

The sci-fi thriller scored a record opening with takings of $76.5m (�37.9m) in its first three days.

It is the best December opening of all time, beating the $72.6m (�36m) start for Lord of the Rings: Return of the King in 2003.