The city is launching an ad campaign discouraging New Yorkers from giving money, food, and clothing to homeless people and asking them to call 311 for help instead.

“Giving money to a panhandler may seem like you’re being compassionate,” Mayor Bloomberg said today at City Hall. “But you’re really not helping that person long term, and just keeping that person going in a life that is probably going to continue to spiral out of control.”

Mr. Bloomberg also announced a 12% reduction in the city’s homeless population, the third straight year it has declined. While Mr. Bloomberg credited new shelter programs from the Department of Homeless Services and cooperation from the Metropolitan Transportation Authority for bringing the numbers down, he said that New Yorkers could do more by halting donations to the homeless, which he said are counterproductive and prevent people from seeking help.

“When you see a homeless person who looks like he or she is struggling, if you really want to do something for that person, pick up the phone and call 311, so that we can send out a professional who can really help,” Mr. Bloomberg said.

The ads, which feature black-and-white pictures of homeless persons, will be featured in subway cars and stations throughout the city. “Give the homeless the kind of change they can really use,” one ad reads.