President Reagan, warning of the increasing "number and sophistication" of espionage activities by the Soviet Union and its allies, yesterday called for reducing the number of Soviet diplomats permitted in the United States as one way to combat spying.

In his first major statement on the subject since the arrests of four Navy men on espionage charges, Reagan said the United States must "reduce the size of the hostile intelligence threat we're up against in this country." He said that 30 to 40 percent of the 2,500 Soviet officials in the United States are known or suspected intelligence officers "and all can be called upon by the KGB."

The State Department in the past has opposed various congressional proposals for parity between the number of Soviet diplomats in the United States and the far lower number of American diplomats in Moscow.

But Reagan, citing the Soviets' "huge advantage," said in his weekly radio address that his administration "intend s to take steps to accomplish" a balance between the two, and to "better control foreign intelligence agents working at the U.N. who have used that organization as a spy nest."

Although he did not mention the Walker case directly, Reagan said, "We need to deal severely with those who betray our country" and to recognize "that spying is a fact of life."

Reagan said officials have "developed a list of things to be accomplished in the counterintelligence and security area" and that he has directed Cabinet officers "to implement the improvements and reforms in every one of these areas on a priority basis."

Reagan said another priority to combat spying is to "improve our own counterintelligence" and "learn from the mistakes of past restrictions which unduly hampered us."

Reagan said the apparent increase in espionage was prompted by Soviet fear of American know-how. "As the West pulled ahead, the Soviets embarked on a major effort to catch up by stealing or buying what they need, from classified information on American satellites, reports on future weapon systems, including our combat aircraft bombers, to our most advanced technologies from high-tech areas like Silicon Valley in California," he said.

"Besides espionage against our most sensitive secrets, theft of the high technology upon which our defense depends, the Soviets have intensified what they call 'active measures' -- propaganda and disinformation meant to mislead Western governments and their citizens, subversion, forgeries and covert action," Reagan added.

He said that Soviet journalists and scholars who appear on television programs "should at least be identified for what they are -- propagandists whose appearances and statements are totally controlled by the Communist Party."

It would be "an injustice to the CIA and a grave mistake," Reagan added, to equate it with the KGB, which he called a "political police operation" whose mission is to "shield the rigid Soviet dictatorship from any internal challenge, to weaken and discredit the United States . . . and to advance the Soviet quest for power to destroy freedom."

Rep. Claude Pepper (D-Fla.), delivering the Democratic response, said the Democrats "strongly support the president in protecting the security of our country, not only against terrorism but against espionage as well."