Bike thefts in Fairfax County are on the rise, and police are urging residents to protect their belongings from opportunistic thieves.

According to authorities, 361 bicycles were reported stolen in the county between Sept. 1 2012 and Aug. 31, 2013 — an increase of about 36 percent over the previous year, when roughly 265 thefts were reported.

The McLean area, police said, appears to have taken a significant hit, with 86 bikes reported stolen.

Many of the bikes were taken when residents left their garage doors open. “Thieves simply seize the opportunity, take the bikes, and walk away,” police said in a news release.

Bike thefts in Arlington County are also on the rise, reaching an all-time high this year, police reported in August.

Authorities are urging residents to take the following precautions to prevent thefts:

• Keep garage doors closed, and don’t leave bikes outside for extended periods of time. “Thieves may be watching and may target ‘unused’ bikes,” police said.

• Invest in a hardened steel U-lock.

• Consider installing motion sensor lights or alarms on your property.

• Take a photo of your bike and record the serial number.

• Register your bicycle on the National Bike Registry.

•Report the stolen item online.