The Egyptian government, claiming broad public support for its action, began a massive crackdown yesterday on critics of the policies of President Anwar Sadat.

The country's "socialist prosecutor" a political watchdog official, announced that investigations have begun of 150 Communists and other dissidents and critics, including Mohammed Heikal, once the country's most prominent journalist.

Sadat told a critical university faculty audience in no uncertain terms yesterday that he means business in the cracdown. He said, however, that there would be no return to the police state system of the era of the late president Gemal Abdel Nasser.

Heikal, a confidant of Nasser and editor of Al Ahram, Egypt's semiofficial newspaper for 18 years, compared Sadat's action to the McCarthyism that swept the United States in the 1950s. The leader of a leftist political party said. "We are living in dictator situation."

The investigation of Sadat critics was announced as the People's Assembly, dominated by his supporters, readled legislation designed to regiment Egypt's fledgling experiment in parliamentary pluralism. Just one week ago, Sadat won 98.27 percent backing for the crackdown in a referendum.

In his university speech, Sadat castigated "language I frankly cannot accept" in two faculty telegrams denouncing the crackdown.

"There will be no going back to concentration camps" he said. "There will be no oppression or Clampdown on the freedom of the individual or expression. But there are limits regulating all this."

Developed over the past four weeks, this theme is being interpreted as meaning that sadat is scaling back his experiment with multiparty democrary which seemed open-ended when it was launched 18 months ago.

Heikal, barred by Sadat from writing in the controlled Egyptain press, has written extensively for publications in Beirut and Amman and was interviewed twice recently by Al Abalt, the paper of the left-wing Porgressive Unionist Party in Egypt.

Heikal has criticized Sadat's domestic policies as well as his peace initiatives toward IsraeL. He told Reuter yesterday, "I differ with President Sadat's views on how to achieve at Middle East peace, certainly, but I thought this was the right of every citizen."

Heikal told reporters: "I hope this is not going to escalate into a McCathy kind of things. There is no need for it."

Heikal was among five prominent intellectuals - four journalists and a poet - who were named by the authorities and informed that they could not travel abroad pending the outcome of the investigations.

Anwar Habib, the socialist prosecutor, said the five men "persisted in sending news dispatches and articles abroad which were defamatory to Egypt and a threat to the security of the home front."

The other four are Mohammend Sid Ahmed, a prominent contributor ot European publications and an Al Ahram editor; dissident poet Fuad Negm; Ahmed Hamrush, an editor at Rose el Youssef, and Salah Eisa, an editor at Al Goumhouria.

A common thread linking the five cases would appear to be their criticism of various aspects of Sadat's policies - including his foundering go-it-alone peace initiative with Israel.

If found guilty by the prosecutor, an office Sadat established in 1971 to dispose of Nasserite plotters accused of an attempted coup, they could become a kind of nonperson.

Draft legislation suggests that they would be banned from any from holding a job of any real influence in the government, trade unions, state industries or any publicly held company.

Khaled Mohieddin, leader of the left-wing Unionist Progessive Party, said "We are living in a dictator situation." Ticking off Sadat's problems at home and abroad - isolation in the Arab world because of the peace initiative, worsening economic and social conditions "and now the feeling we are not living in democracy" - he said that if Sadat "could solve even one of these problems maybe he could survive.

Sadat has made it clear that the purge is directed against Communists - traditionally banned in Egypt - as well as Nasserites and politicians now in the resuscitated Wafd party which ran Egypt before Nasser's 1952 revolution.

Among the journalists under investigation were 22 others working in Egypt whose names have not been released and 34 working abroad in Europe, Iraq and Libya. They have been asked to return home or face trials in absentia. Few are expected to return.

The remaining 90 Egyptians under investigation were described as Nasser era politicians, but were not named.

Until the prosecutor's announcement, the crackdown had been limited.

The most prominent case involved the 60-day imprisonment of leftist member of parliament Abu el Ezz Hariri, despite his parliamentary immunity. The socialist prosecutor said immunity could be lifted if, as he claimed was the case, a member of parliament had committed acts detrimental to state security.

Also purged were two leftist politicians who were removed from prominent trade union jobs. The Unionist Progressive Party's weekly newspaper, Al Ahali, has had its last two issues confiscated - the first for advocating a "no" vote in the referendum and the second for suggesting that the results of last week's referendum were doctored.

The crackdown against Communists now involves about 70 Egyptians arrested in the last week.