The "Twilight" co-stars were seen mingling with the other guests in the VIP section but later managed to sneak out for an alone time on the terrace.
The duo were then pictured getting intimate and kissing for a long time, reported Aceshowbiz.
"They kissed several times. Earlier that night they enjoyed their own private karaoke session singing 'Hit the Road Jack'," a source said.
The couple have always dodged questions about their love life. When asked by a reporter recently whether they were engaged, Stewart said, "Everyone wants to know. Everyone knows already - it's ridiculous."