A cigarette sparked the fire, General Manager Steve Vallarelli said today.
The Italian family restaurant at 95 Turnpike Road (Route 9) was full at the time, he said.
"We had to evacuate," Vallarelli said.
"The smoke came into the restaurant. So, we immediately called the fire department, got the patrons outside."
Chateau officials are "still assessing the damage," Vallerelli said.
The Westborough Fire Department received a 911 call from the restaurant at 7:07 p.m.
"We couldn't really determine what it was, because the smoke came in so quickly. So, we thought it might be something in the basement. Luckily, it wasn't," Vallarelli said.