(Corrects that 11,000 households urged to evacuate but number that left was not known)

BOULDER, Colo., Jan 7 (Reuters) - Three windblown wildfires have destroyed at least two structures, jumped a major road and forced the evacuation of homes outside Boulder, Colorado, fire authorities said on Wednesday.

More than 75 firefighters from several agencies are battling the fast-moving flames and no injuries have been reported, said Patricia Demchak, spokeswoman for Boulder County.

One of the blazes started in dry grass after high winds downed a power transformer on the outskirts of Boulder, about 30 miles (48 km) northwest of Denver, she said.

The cause of the other fires is unknown.

Firefighters estimate the largest fire to be over 100 acres (40 hectares), but embers from the blazes have started spot fires that have been extinguished quickly by fire crews as they try to protect two threatened subdivisions.

Officials notified residents of 11,000 homes in the Boulder area that they should evacuate but said it was not clear how many people left. Several hundred turned up an emergency center set up at a local high school, they said.

Sustained winds of 35 to 50 miles per hour (56-80 kph), with gusts up to 80 mph (129 kph), have hampered firefighting efforts.

Roads in the area have been closed in the semi-rural area that is home to several horse and livestock ranches. An evacuation center for large animals "has completely filled up," said county spokeswoman Barbara Halpin. (Editing by Peter Cooney)

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