The leaders of Rhodesia's Patriotic Front guerillas, Joshua Nkomo and Robert Mugabe, yesterday denounced Prime Minister Ian Smith's plan for a transfer to black majority rule as a fake, and declared the internal settlement "null and void."

The two men, who command thousands of Rhodesian guerillas based in Zambia and Mozambique, also denounced the three black leaders who joined Smith in signing the internal agreement last week, and refused to meet with one of the three, Bishop Abel Muzorewa, who is currently visiting the United States.

As a result, it appeared that U.S. Ambassador Andrew Young and British Foreign Secretary David Owen, who conferred with Secretary of State Cyrus Vance yesterday in Washington, would hold talks today in New York with Nkomo and Mugabe.They have been seeking to bring the three black leaders together for negotiations aimed at heading off the prospect of continued guerilla war in Rhodesia.

Nevertheless, there were indications that the positions of the two Patriotic Front leaders might be more flexible than their rhetoric would indicate.

Both Nkomo and Mugabe postponed plans to present their arguments against the internal settlement before the U,N. Security Council yesterday, and source suggested that they had been persuaded to await the outcome of today's talks with Owen and Young before formally addressing the world body.

Young, who had a working breakfast with Nkomo Tuesday and a working lunch with Mugabe yesterday, said he felt progress had been made in the consultations and that there was room for maneuver in the position of the Patriotic Front leaders.

Nkomo, in joint news conference with Mugabe, charged that Smith had "contrived" with the three black leaders who took part in the internal settlement to create the appearance of majority rule.