Joshua Nkomo, coleader of the Patriotic Front guerrilla alliance and a man under frequent death threats from angry whites, returned to Rhodesia today from three years in exile in Zambia to an enthusiastic welcome by 150,000 supporters and urged an end to hostilities.

Calling the country's bloody, seven-year war "a tragedy" and "regrettable," he told a press conference: "Having fought, let us say to each other, it's over . . . . The war is over."

He praised the London agreement signed that month to end the war and bring the country to black-majority rule-and added:

"Let every man or woman of every color or creed now say this is an opportunity we must seize."

Nkomo, who has been backed by Soviet arms in the war, took the high ground in opening his campaign for late-February elections to be run by the new temporary British colonial administration. He repeatedly stressed the theme that there must be reconcilliation between blacks and whites, between the warning military forces and among the differing tribes and sections.

Under tight police security, Nkomo was taken from the airport by helicopter to a rally outside Salisbury, where more than 150,000 supporters gave him an enthusiastic welcome. Police estimated that about 65,000 were from outside the Salisbury area, on the basis of almost 300 buses and three trains used to transport people mainly from the Bulawayo area, the southwestern stronghold of Nkomo's Ndebele tribe.

The crowd was about two to three times the number that showed up at the same grounds last week for Bishop Abel Muzorewa, the former prime minister who stepped aside for the British colonial administration.

Nkomo made no reference in either his press conference or his speech at the rally to Muzoreqa, a bitter political opponent who has said election of Nkomo or Robert Mugabe, his Patriotic Front partner, would lead to a Marxist government and chaos.

Nor was there any mention of the issue that made Nkomo public enemy number one among many of the 220,000 white minority -- the shooting down by his guerrillas of two commercial airlines in 1978 and 1979, killing about 100 persons.

Numerous whites who lost relatives in the attacks have threatened to kill the Zimbahwe African People's Union.

While Musarurwa, Nkomo's spokesman, said the party's headquarters just yesterday received in the mail a clipped-out pro-Nkomo newspaper ad on which a hole, ringed in red, had been cut in a picture of Knomo's head. Next to the hole was the nomenclature of a rifel commonly used by Rhodesian whites and below the picture the anonymous sender had written: "He'll be dead within two weeks."

Nkomo discounted the threats on his life, saying, "I'm going to die some day. Why worry about it?

He said the cause of Zimbabwe, the African name the country will take upon independence, "is far bigger than my life. If I die, Zimbabwe is not dead."

Nkomo arrived from Lusaka, Zambia, almost three hours late, held a brief press conference and then was taken by helicopter to the African township of Highfield outside Salisbury for the rally where the boisterous crowd was waiting for him in the boiling sun.

Nkomo had wanted to have a motorcade but police insisted that he use a helicopter, rented from the Commonwealth cease-fire monitoring force, because of the security risk.

Even though he acknowledged at his press conference that Mugabe has so far declined to run as a coalition under the banner of the Patriotic Front, Nkomo gave emphasis to the Front and not his own organization. Many of his followers wore T-shirts reading, "PF the liberators" and "PF for peace."

All the posters and banners on the speaker's platform cited the Patriotic Front, saying it stod for unity and stability. It was an obvious attempt to deal with Muzorewa's criticism of the split between Nkomo and Mugaabe, who is expected to return next Sunday frm five years of exile in Mozabique.

Although the crowd was far larger and more enthusiastic than for Muzorewa's campaign opener last Sunday, observers noted that it was too early to draw conclusions from the figures. Muzorewa has predicted that at first, the Front will draw crows of curious onlookers who are not necessarily supporters but want to see the exiled officials -- some of whom have been outside the country for more than 15 years.

In his press confrence, Nkomo voiced disappointment about some aspects of the British administration. He particularly objected to the continued presence of South African military units in the country and the use by the governor Lord Soames, Rhodesian security forces against guerrillas who have not assembled as required by the London agreement. He also called for an increase in the size of the cease-fire monitoring force from 1,300 to at least 5,000.

He was careful, however, not to condemn the governor's actions, and he said he wanted to talk to Soames first. Nkomo said some actions, like the killing of seven of his guerrillas Thursday could have been a mistake.

Nkomo hammered repeatedly at the theme of reconciliation and sought to reassure the whites.

"I ask all Zimbabweans, let there be no recriminatory statements that bring about misunderstanding."

He said he attached no importance to color and added: "All the people who have chosen to make this country their home, it is their home. They don't have to ask anybody to be here."