Fire crews spent four hours battling the "significant" blaze at the restaurant in Troon.
The alarm was raised at the Lido cafe and bar on West Portland Street at around 11.30am yesterday.
About 30 firefighters and five engines were in attendance at the height of incident – and crews were still there today to ensure there was no further outbreak of fire.
Shocked diners and staff were evacuated from the busy eatery when the fire broke out.It quickly spread and soon the building was engulfed in flames which could be seen shooting from the roof.
No one was injured and the fire was brought under control by around 3pm.
It is believed the blaze may have started in the kitchen of the restaurant, but a joint probe between the police and fire service has been launched into the cause.
Firefighters could be seen standing on the roof of neighbouring buildings assessing the situation.
Crew members entered the building wearing breathing apparatus and managed to prevent the flames from spreading.
An aerial rescue platform (ARP) was used to tackle the rising flames on the roof.
A Strathclyde Fire and Rescue spokeswoman said: "We received a call at 11.26am to a fire in West Portland Street, Troon.
"The first two engines, which came from Troon, arrived to a significant fire which had already started breaking through the roof of the local cafe/bar.
"At the height of the fire there were five appliances in attendance, with more than 30 firefighters.
"They were attacking a very well-developed fire wearing breathing apparatus and using main jets which prevented the fire from spreading and escalating out of control."
Fire crews from Kilmarnock and Ardrossan were at the site today.
Drivers faced road diversions after the streets surrounding the blaze-hit restaurant were shut.
West Portland Street, Shore Road and Academy Street, were all closed in both directions and remained cordoned off today.
The Lido is one of six restaurants owned by entrepreneur Colin Blair, from Kilwinning, operated under his Buzzworks chain, the others being Scotts in Troon and Largs, Elliots and Dome in Prestwick and the Treehouse in Ayr.