Former CBS anchorman Walter Cronkite lashed out at the Reagan administration yesterday, describing a "pattern of restriction" he said it was imposing on the nation's press, and comparing its attitude toward American journalists in El Salvador to the hostility of U.S. officials in the Vietnam war.

In a provocative speech before the annual National Association of Broadcasters' convention in which Cronkite accepted the organization's distinguished service award, the retired newscaster said he was "greatly worried" about the intrusions he contended the Reagan administration wanted to make on the freedom of the press in a number of different areas.

The administration's actions do "not give us any great reason for press security," he said.

Chief among Cronkite's concerns were the administration's official reports on events in El Salvador, which he said reminded him of the Vietnam war, when government officials used "tortured wishful thinking, manipulation of figures and other distortions" to make their case.

"Today I very much fear we may be in for more of the same. I do not intend to liken El Salvador to Vietnam in any way . . . except in this: That official reports and explanations often are woefully unconvincing, transparently wrong and often in conflict with reports from experienced and reliable American reporters on the scene.

"And we already are hearing echoes of that earlier battle between press and officialdom. The reporters are 'naive,' 'romantic,' 'leftist,' 'subversive,' 'anti-American.' 'They don't or won't understand the big picture . . . Why the hell can't they get on the team.' Yes, we've been down this road before."

But Cronkite added that his concerns about press freedom grow as well from "all the little restrictions and constrictions enacted and proposed on the people's right to know" by the administration.

"None of these proposals and initiatives may seem all that serious a matter in itself, but taken together, they form a pattern which we should all be worrying about. It is a pattern of restriction. It is the solution of those who feel America has become too open a society and needs to be closed off some."

Among the administration's restrictions specifically cited by Cronkite are:

* The proposal to limit the Freedom of Information Act by restricting the type and amount of government material available to the public.

* A proposal to protect a wide range of business information from public review, such as information about dangerous food and drugs, unsafe consumer goods, deceptive advertising, and pollution and environmental threats.

* A reversal in a 30-year government policy of automatic declassification of historic government documents. This is "a serious blow" to the age-old business of trying to look in the past to protect the country in the future, he said.

* A request to universities to restrict access to certain scientific courses that could benefit potential enemies of our country and a requirement that scientists clear in advance with the government any project that deals with government secrets or threatens the nation's security. This represents a "ghastly restriction of academic freedom," Cronkite charged.

* A proposal to copyright government information so users must pay royalties or fees.

* The agent's identity bill which will make it illegal for anyone to reveal any information about Central Intelligence Agency employes, no matter what their actions may be.

* The administration's support for two Supreme Court rulings that make it legal for police officers to raid newsrooms in search of sources and also compel journalists to disclose sources in court, on pain of jail sentences.

Cronkite urged that journalists fight these restrictions and defend a "fair and vigorously active news media and its access to information a democratic public needs to know. That remains democracy's only fail-safe system against both the dangers of its own excesses and the approach of tyranny."