In a sudden crackdown on opposition to the new regime, the Spanish police arrested nearly 300 political dissidents yesterday, including the recently freed leftist labor leader. Marcelino Camacho.

The massive show or police power, the biggest in years, undercut new King Juan Carlos's efforts at liberalization and his pardon order freeing Camacho and a few others serving sentences for "political offenses."

Went to Get Newspaper

The Associated Press reported that the policemen picked up the 57-year-old Camacho near his home as he went to buy a Sunday newspaper. They have so far declined to specify charges and denied his wife and lawyer permission to see him.

Juan Marichal, professor of Romance Languages and Literatures, said yesterday that this crackdown was symbolic of "a last fight by the remaining fascists to prevent change" and added that it would do little to stave off an eventual collapse of the right.

William Watson, associate professor of history at MIT, said yesterday that the police action was a result of Spain's "dictarchy" fearing reaction from right wing former Franco supporters.

Within minutes of Camacho's arrest the police began seizing anyone considered suspicious within a half-mile radius of Madrid's Carabanchel Prison, where the illegal Communist Party had called a demonstration to demand amnesty for the rest of Spain's estimated 2000 political prisoners. The demonstration never started.

300 Picked Up

Police declined to give the number arrested, but the opposition lawyers said more than 300 persons were picked up. Police also fired smoke grenades, hit people in cafes, and warned newsmen away from the prison area. An ABC-TV crew was handcuffed and held for four hours.

When Camacho was released from the prison last Sunday, he declared that he would take his fight for amnesty of all political prisoners to the streets. He called the king's pardon an insult, because, he said, it left 90 per cent of the country's political prisoners behind bars.

The 37-year-old monarch's pardon was considered a gesture to appease Spain's powerful political opposition that was outlawed during 36 years of rule by Gen. Francisco Franco, who died on Nov. 20.

Watson said that the pardon was an "attempt to achieve popular support" for Carlos's government.

He said that to gain popular support, the government must reform, but the government is caught between a right-wing faction of the leadership and the majority of the Spanish people, who want democracy.

Marichal also said that most of the people in Spain want a "real peace" and an "amnesty of all political prisoners." He considers the present monarchy to be merely a "transitional stage."

The crackdown also came as conservative Premier Carlos Arias Navarro, confirmed by the king two days ago to continue as head of government, prepared to pick a new cabinet.