HARARE, ZIMBABWE, DEC. 22 -- Seven years after overthrowing Ian Smith's white Rhodesian rule, Prime Minister Robert Mugabe and his fellow liberation leader, Joshua Nkomo, today ended their longstanding rivalry and officially united their two parties.

Creation of a one-party state, a goal promoted by the Marxist Mugabe, removes Zimbabwe as one of only a half-dozen sub-Saharan African countries with a legal political opposition.

Both men said they hoped that unifying the parties would end violence that has plagued the southern Matabeleland provinces since independence. Matabeleland is the home of the Ndebele tribe, which Nkomo's party largely represents.

"We hope that those who yesterday felt they had cause to wage a political fight because we were divided will lay down their arms," Mugabe said.

Under the agreement, signed in a champagne ceremony that ended with the two men embracing for the cameras, the new organization will bear the name of Mugabe's ruling party -- the Zimbabwe African National Union (Patriotic Front), or ZANU(PF) -- and do away with Nkomo's opposition Zimbabwe African Peoples Union (ZAPU).

The merger brings Nkomo -- who joined the liberation struggle earlier than Mugabe and is considered "Father Zimbabwe" by many here -- back into mainstream Zimbabwean politics for the first time in five years. He had been Mugabe's first minister of home affairs, but was fired in 1982 after arms caches were found on his property.

The agreement, which names Mugabe as president and first secretary of the new party, allows for two party positions titled vice president/second secretary. Nkomo is seen as likely to get one of those posts.

He also is considered likely to become second vice president of a new government to be formed Dec. 31, when Mugabe becomes executive president with expanded powers.

Several former ZAPU members will be chosen for Cabinet positions, as well as posts in the civil service and diplomatic corps, Mugabe said in a speech following the signing.

Beginning in 1985, the two parties held 10 meetings aimed at unifying and at eliminating tribal divisions from national politics. The two rivals were divided by no discernible ideological differences, but their representation in Parliament, 80 to 20, exactly reflected the proportion of Mugabe's Shona tribesmen to the Ndebele.

The government considers former ZAPU liberation fighters to be the core of the "dissident" movement in Matabeleland, where violence reached its peak this year. Nine white farmers have been murdered since May, when unity talks first collapsed.

On Nov. 27, 16 white missionaries and children, including two Americans, were massacred on a farm near the southern city of Bulawayo. It was the bloodiest incident of antiwhite violence since independence. The killers left behind a note criticizing Mugabe's policies toward ZAPU.

Until October, the government had been accusing ZAPU of links to the killers of the farmers. ZAPU offices were closed, its meetings banned and several of its officials were briefly detained.