Mayor Bloomberg yesterday defended higher tolls on Port Authority crossings, saying they’re necessary to keep the bridges from tumbling down.

The mayor conceded that the jacked-up tolls — rising from $8 to $12 for those paying with cash starting tomorrow — would have an impact on the city’s economy.

The tolls will “certainly hurt some businesses and some people,” Bloomberg said on his weekly WOR Radio show.

But he quickly added that the alternative could be disastrous.

“On the other hand, if one of the bridges fell down, how would you feel about that for the economy?” the mayor said.

Motorists at PA crossings — including the George Washington, Bayonne and Goethals bridges, the Outerbridge Crossing and the Lincoln and Holland tunnels — are being hit with a 50 percent hike if they use cash. E-ZPass holders are getting a $1.50 increase to $9.50 during peak hours.

“If you want them to provide services, they’re going to have to get the money from someplace,” the mayor added.

“It’s easy to . . . say it’s an outrage. When you have a better source, let me know.”