Nine senior officials of Joshua Nkomo's Zimbabwe African People's Union were arrested today as Prime Minister Robert Mugabe cracked down on the party of his nominal coalition partner.

Those arrested are the first persons detained by the Mugabe government for political acts. Ironically, all had been arrested previously by the white-minority government of Ian Smith against which the two parties joined forces in a seven-year guerilla war.

The move against Nkomo's party was regarded in many quarters as the first overt step moving the newly independent nation toward a de facto one-party state, a basic tenet of Mugabe's policy during the war.

Neither the government nor the police would give any details of the arrests, which were announced by Nkomo at a press conference. As home affairs minister, Nkomo is technically in charge of the police.

Nkomo complained that the arrests "happened without my knowledge even in my capacity as minister of home affairs." The detentions, he said, were ordered by Emmerson Mnangagwa, minister of state in the prime minister's office and head of intelligence for Mugabe.

Nkomo, the father of the nationalist movement in Zimbabwe but now relegated to the role of leading a minority party, also told the press that the arrests came just hours after he and Mugabe had a "very open and frank" two-hour meeting yesterday to discuss "very serious developments" in the country.

This was a reference to recent clashes between former guerrillas loyal to the two parties. In the most serious incident, 58 persons, mainly civilians, were killed and hundreds were injured last week in a black township outside the southwestern city of Bulawayo when the two rival groups fought day-long battles with automatic weapons, rockets and grenades.

Today's arrests came on the eve of provincial elections providing the first chance to elect black-majority governments in some localitis here after 90 years of white rule.

Mugabe's Zimbabwe African National Party has made the elections a partisan test, which has led to outbreaks of violence. There have been sporadic clashes and explosions since the Bulawayo incident, and the arrests could lead to further violence in the heated election atmosphere.

Although the government refused to comment on the arrests, Mugabe supporters said the move was in retaliation for the Bulawayo violence, which they claim was started by Nkomo's party.

One Mugabe supporter said he was surprised there were not more arrests, including three members of parliament from Bulawayo whom he accused of fomenting last week's strife.

Nkomo said that five of the nine arrested were member of his party's Central Committee, including Sydney Maluga, a member of Parliament, and Information Secretary Mark Nziramasanga, who was taken from his home in Salisbury while the rest were arrested in Bulawayo.

Nkomo said he understood the arrests of his party members were continuing.

The party leader said the meeting with Mugabe was "at my own instigation." The two leaders, he added, had agreed to hold a joint meeting of their two central committees to try to iron out difficulties. It remains to be seen whether the meeting will still be held.

Nkomo went to see the prime minister again this morning after learning about the arrests but said Mugabe provided no reason for the detentions except for Nziramasanga. He was accused of sharply criticizing the government in a Radio Moscow broadcast.

"I don't listen to radio," either domestic or international, "these days because I don't want to be annoyed," Nkomo told the press conference. His party has frequently complained that the government radio is biased in favor of Mugabe.

Just yesterday Nziramasanga issued a press release accusing Mugabe's party of "an endless chain of barbarous acts of violence" and failing to establish peace and order.

Nkomo appealed "to everyone to remain calm while all effort is being taken to sort out the problem." He refusted to speculate on whether the government was trying to crush his party or whether he would pull out of the alliance as has been frequently rumored.

He bitterly denounced the removal of the special branch police from his control as home affairs minister. Mugabe put the special police branch under the command of his intelligence specialist, Mnangagwa.

"We are now face-to-face with a political police," Nkomo said.

One Mugabe aide called the arrests "political reality." Describing the situation after the Bulawayo violence as "critical," the official said, "If Nkomo were in charge he would have done the same thing."

Another source said the Bulawayo incident had helped to rally support behind Mugabe. He indicated that moves were under way to disarm the more than 20,000 former guerrillas from the two factions who are waiting to be integrated into the new national army.