MOSCOW, OCT. 12 -- In banter on the streets of Leningrad today, Soviet leader Mikhail Gorbachev gave a lively defense of his policy of seeking closer ties with Washington.

Asked by a young Russian whether he would be meeting with President Reagan, Gorbachev said, "We need a dialogue." The Communist Party chief has agreed to meet Reagan in the United States, with the session expected this year.

"There are many problems in the world that cannot be solved without the participation of the Soviet Union and the participation of America," Gorbachev said. "That's why the dialogue is necessary."

For his latest in a series of encounters on the streets of Soviet cities, Gorbachev flew to Leningrad this morning. He chatted with passers-by at various monuments to the Russian Revolution, which broke out there 70 years ago. The encounters were broadcast nationally on the evening news.

During one back-and-forth with a crowd of workers and students, Gorbachev also called for increased trade with the United States:

"We now do practically no trade with the U.S. We exist without it, and they do, too. But a new international situation cannot be built without the participation of both countries. So we need a dialogue of trade, of meetings, so that our peoples also can reach an understanding."

Gorbachev also used the meetings to plead for continued support of his reform policies. "We will not retreat from what we have decided," he said.

Gorbachev also spoke briefly to World War II veterans and elderly members of the party.

The appearances were widely viewed here as the kick-off for celebration the 70th anniversary of the revolution, which is to climax with a gala jubilee here early next month.