The fire began around 8 a.m. at the moving and storage facility at 2200 El Camino Real near Woodside Road.
The streets are both main thoroughfares in the area that were shut down for much of the day but were in the process of reopening as of 4:15 p.m., San Mateo County emergency officials said.
Fire crews from Redwood City, Menlo Park and Woodside sprayed water for several hours on a 1,000-gallon propane silo that had been burning and leaking propane gas since about 8:30 a.m., Redwood City Deputy Fire Chief Stan Maupin said.
The process has been considered "controlled," meaning fire fighters were monitoring the temperature of the tank, its internal pressure and the fire burning below.
A steady stream of water -- anywhere from 1,000 to 2,000 gallons per minute -- was being used to mitigate further risk, Maupin said.
A report came into the fire department at 8:20 a.m. about an explosion at the site.
Arriving crews found a small personal-use propane tank that had exploded and had threatened the larger tank. A U-Haul employee was apparently filling the smaller tank when the incident occurred.
Maupin said that based on information provided by the propane technicians, it is estimated that about 500 gallons of propane were inside the tank when the fire started.
Just after 11:20 a.m., a rise of orange flames and black smoke was visible, and moments later the San Mateo County bomb squad lent fire officials its remote robot to give fire officials a better look at the source of the fire.
Otherwise, only two powerful hoses spraying water on each side of the white silo could be seen amid the five engines, one ladder truck and a host of police caution tape cordoning off sections of busy El Camino Real.
Baldez Mann, who owns the Chevron gas station directly across the street from the U-Haul facility, was working the morning shift when he heard a bang and saw a plume of black smoke rise into the air.
"I heard a big boom and then black flames went up more and more," Mann said.
He said he smelled nothing but was startled when he heard the fire engines.
El Camino Real was closed between Center Street and Oak Avenue in both directions, and Woodside Road was closed between Middlefield Road and Hudson Street due to the response to the fire.
Redwood City residents were also asked to shelter-in-place in the area while crews cleared the scene.