SAN FRANCISCO -- As many as 100 people were unable to go home Monday night after a four-alarm fire engulfed two adjacent apartment buildings in San Francisco's Mission District and forced the evacuation of St. Patrick's Day revelers at the historic Irish pub the Dovre Club.

At around 6 p.m., flames broke out in a multi-unit residential building on Valencia Street near 26th Street. Firefighters say they don't know how it started, but the blaze quickly spread next door to another apartment building. The heavy smoke also filled a third building.

"The structure was fully involved on the fourth floor. It's a whole lot of fire," said SF Fire Department Lt. Ken Smith.

130 firefighters went to work battling the flames but they say their priority was evacuating the many residents in the two engulfed buildings as well as surrounding buildings.

The Contreras family told KTVU they feel fortunate to have survived.

"We heard screaming. My father told us to get out. We checked the sky and it was all smoke," described Angelica Contreras.

Firefighters say they had a tough time trying to contain the flames. After a fourth alarm was called at 6:36 p.m., it took more than an hour before crews got the upper hand on the fire.

They say flames spread quickly thru the wooden frames of the buildings. The large size of the structures -- 80-feet deep and four-stories high -- made battling a challenge.

Located just two doors down from the fire is the Dovre Club. The historic Irish pub was packed with patrons for St. Patrick's Day who had to be evacuated.

"[There was] just a lot of black smoke and a lot of flames. It was probably the worst fire I've seen in my life," said Dovre Club bartender Stephanie Crebo.

Red Cross workers are helping displaced residents find shelter. One resident told KTVU she was relieved to come home find her daughter uninjured, but she doesn't know what's left of her apartment and her belongings.

"I'm nervous. I don't know what's going on inside. We need to find out," said an anxious Maura Lechado.

Firefighters will remain on the scene for hours to keep an eye on the buildings and ensure that no hot spots flare up to restart the fire.