put on a happy face Monday night, at the afterparty following the New York City premiere of Cosmopolis
Making his way to the PH-D Rooftop Lounge at Manhattan's Dream Downtown hotel at around 10 p.m., the heartbroken actor
– who was recently cheated on
by girlfriend Kristen Stewart
– sipped a bottle of beer and took regular puffs from an electronic cigarette, according to an onlooker.
"He was very charismatic, all smiles, looking very relaxed and hugging all who stopped by to say hi," the insider tells PEOPLE about Pattinson, who left around midnight. "The general crowd gravitated towards his table, but he was shielded by security. He was surrounded by women and the movie's cast."
Adds the source, "The whole room was watching him, and he looked genuinely at ease and comfortable the whole time."
Other guests at the fete included Judah Friedlander, Shia LaBeouf
and the film's costar Paul Giamatti, who stayed "away from the cast, looking over at Rob, and looked like he felt sorry for him," the source says.
Earlier in the day, Pattinson did his best at cracking jokes with Jon Stewart during a taping
of The Daily Show
. – Kristin Boehm