Nelson Mandela, imprisoned leader of the black South African underground, would refuse to join a constitutional convention with the white minority government if it released him, his wife, Winnie Mandela, said today when she defied restriction orders to address a press conference here.

Winnie Mandela also gave a sharp snub to the Reagan administration, refusing to accept a $10,000 State Department donation to help rebuild her house and clinic, which were firebombed last week, while accepting a $6,000 donation from 12 U.S. senators. She said the administration was giving tacit support to South Africa's apartheid system of segregation.

"The time for talking about a convention is past," she declared. "The only thing that is left to be discussed by the people of this country and the ruling white Afrikaners is the handing over of power to the black majority."

Mandela's appearance at the press conference was her third act of open defiance in a week of stringent restriction orders that the government has used to banish and silence her. If prosecuted, she could be imprisoned for nine years.

The restriction orders prohibit her from being in the company of more than one person at a time and confine her to the small country town of Brandfort, 250 miles south of Johannesburg. She left the town after the firebombing and has refused to return until the authorities restore her burned-out house. Today's was her second press conference since coming to Johannesburg.

Plainclothes security policemen in two cars parked outside the building where the press conference was held but made no attempt to intervene.

Through her defiance Mandela, 52, appears to be trying to move into the black leadership vacuum created by the arrest on treason changes and detention under emergency regulations of nearly the entire top structure of the United Democratic Front, the main black nationalist movement still legally permitted.

Mandela seemed to be presenting herself today as a surrogate for her husband, who is widely acknowledged as the country's major black leader though he has been in prison for 23 years. Nelson Mandela was sentenced to life imprisonment for plotting to commit sabotage and advocating the overthrow of the government.

In acting as his surrogate, she appeared to be challenging the government's claim that it wants to release him but that he is "imprisoning himself" by refusing to renounce his movement's commitment to overthrow white minority rule by guerrilla struggle. The government has also indicated that Mandela must accept citizenship in the Transkei, one of the 10 tribal "homelands" it has established, as a condition for his release.

Implicit in Winnie Mandela's actions is the idea that if she is imprisoned for expressing her husband's views, it will amount to an acknowledgment that Nelson Mandela is being kept in prison to silence him.

Although the government has taken no action against her as yet, Justice Minister Jacobus Coetsee reflected official anger when he said in Pretoria later today that he might curtail family visits to political prisoners because Nelson Mandela had used such a visit to convey his views on a constitutional convention.

"No state can permit political propaganda to be issued from prison institutions," Coetsee said.

However, the government seemed to adopt a different attitude when it arranged for Moral Majority leader Jerry Falwell, accompanied by an associate and a columnist for The Washington Times newspaper, to see Mandela in prison this week for an interview.

In an interview after the press conference, Winnie Mandela accused the authorities of applying double standards. "They say I shouldn't talk politics to him and they won't allow most of the international press to see him, but then they prescribe some visitors when they think it will suit their purposes," Mandela said.

Winnie Mandela's announcement of her husband's refusal to participate in a constitutional convention appeared to be a response to Thursday's hard-line speech by President Pieter W. Botha, in which he failed to announce expected reforms and repeated his refusal to release the ANC leader unconditionally.

Although the government is officially opposed to the idea of a constitutional convention, it has said it will negotiate with black leaders about constitutional reforms and there is growing pressure for it to release Mandela and include him in such negotiations.

His rejection of the idea dismayed some liberals. Frederik van Zyl Slabbert, leader of the opposition Progressive Federal Party, which advocates a convention of all racial groups to write a new constitution, said today that he would ask Botha for permission to see Mandela in prison to discuss the issue.

Winnie Mandela said Botha had made clear in his speech what negotiations with him would be like.

"He says he does not want to prescribe political solutions for our people, but at the same time he excludes all options which fall outside the parameters he has laid down," she said. "It is quite clear that he will negotiate only with those so-called leaders who are prepared to talk about the reform of apartheid, not its removal."

She read a letter from her lawyer to the State Department rejecting its offer of $10,000 from a human rights fund to help rebuild her house and clinic.

The letter accused the Reagan administration of supporting the Pretoria government and said Mandela had been "greatly disturbed" by statements that President Reagan had made after the police shooting of 20 black mourners at a funeral last March and after Botha's declaration of martial law last month.

There were continued reports of unrest today, with one more person killed, a month after the government declared a state of emergency in 36 towns and cities. Police reported that a black man was stabbed to death in Pietermaritzburg's Imbali township, and about 100 students were arrested in the Cape Province town of Oudtshoorn after unrest at a teachers training college.

Meanwhile, a lawyer for Benjamin Moloisi, who won a three-week reprieve Tuesday from execution for the 1983 murder of a black policeman, said her client signed an affidavit Tuesday admitting he took part in the killing but did so "under pressure from trained ANC guerrillas," Reuter reported.