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A four-alarm fire tore through a commercial building in downtown Palo Alto Sunday night and took more than five hours to extinguish, according to fire officials.

University Avenue is closed between Bryant and Florence streets and Bryant Street is closed between University and Hamilton Avenue as fire personnel continue to monitor "hot spots" and investigate the scene.

The fire at 300 University Ave. was originally reported at 9:35 p.m. Sunday night and had dozens of onlookers gathered on the street.

Flames could be seen bursting from the roof of the Walgreens building well past midnight. As of 1 a.m., dense clouds of smoke still billowed from the vintage downtown structure despite two ladder trucks and some ground units pouring steady streams of water at the flames.

The fire was concentrated on the second floor of the building, in offices that had been occupied until recently by the San Jose Mercury News' North County office.

Palo Alto Police Agent Dan Ryan said that a hole eventually burned through the roof of the building, enabling firefighters to pour water directly onto the flames.

By 10:30 p.m., with flames shooting 20 to 30 feet into the air, 13 engines and trucks from Palo Alto, Mountain View, Menlo Park and Santa Clara County had responded.

No people were believed to be in the building when the fire broke out and no one has been reported injured.

Witnesses reported seeing windows on the second floor facing University Avenue blow out and send glass showering down on the sidewalk below as fire units arrived around 9:45 p.m.

The Subway restaurant on University Avenue was open when the fire broke out and was evacuated.

Walgreens closed for the evening at 9 p.m.

Firefighters became concerned when flames and embers began blowing across Bryant Street toward other buildings on University Avenue and moved the crowd of onlookers back a block from the scene.

One early witness to the fire was alerted by the smell of smoke while he and a friend were walking on Hamilton Avenue at Bryant Street, although pedestrians were allowed as close in as the University and Bryant intersections for a time. They were later moved back a quarter block.

"It smelled like someone barbecuing, or bad fireworks, but we didn't hear any bang," according to witness Herbert Freeman, a Mountain View resident. He said he and his friend, Mac Bennett, of Palo Alto flagged down a passing California Highway Patrol vehicle and said they thought there was a fire nearby.

But Ryan said an alarm initially alerted officials of the fire.

The Walgreens building and PAIX (the Peering and Internet eXchange) are separated by a narrow alley. PAIX has been a worldwide hub for Internet traffic and is operated by Switch and Data, which provides Internet exchange points in Palo Alto, New York City and Dallas, among other places.

Shortly before 11:30 p.m., a fire official evacuated all fire personnel from the building and warned of a 200-foot "collapse zone." Soon thereafter the hole opened in the roof, allowing water to reach the flames.

This story was updated at 1:30 a.m. Monday, July 2.

— Palo Alto Weekly staff