The Patriotic Front guerrilla alliance today rejected a draft constitution for an independent Rhodesia that Britain said cannot be changed in any significant way.

Instead the Front, in a 34-page document, proposed major changes that give ground on a couple of issues but hold firmly to the Front's positions in several key areas that Britain had turned down in four weeks of negotiations.

The Front's move leaves it to British Foreign Secretary Lord Carrington to determine where to go from here. This ninth Rhodesia conference is generally regarded as the last realistic chance to get a negotiated settlement to the intractable 14-year-old independence problem.

Carrington said he would respond tomorrow to the Patriotic Front objections to the British-proposed constitution. It has been accepted conditionally by the Zimbabwe-Rhodesian government.

Joshua Nkomo and Robert Mugabe, coleaders of the Front, told a press conference they were not about to walk out of the conference and suggested that negotiations continue.

Nkomo said the Front had "gone a long way" and had "reduced points of difference to a minimum." British sources acknowledged the reasonable tone of the guerrillas but said a first glance at the document showed wide areas of disagreement.

The British maintain that their constitution comes down between the two warring sides and calls for compromises from both.

Carrington bluntly told the delegates: "It would be misleading the meeting if I suggested any realistic prescription for agreement could be reached on the basis of any document other than that put forward by Britain."

Carrington had originally asked both sides last Wednesday to accept the British proposals at today's meeting in what was seen as a "take-it-or-leave-it" offer.

British spokesman Nicholas Fenn, questioned by reporters, said he did not expect the conference to break down tomorrow but admitted that was a possibility.

Instead, he cast a somewhat longer time frame, saying, "Lord Carrington has made it clear that he regards this week as a time for decision."

If the conference can agree on the constitution, it still faces the more difficult hurdle of working out transitional arrangements, including a cease-fire in the seven-year-old guerrilla war, disposition of the military and elections to implement the charter.

Mugabe and Nkomo called today for the conference to go on to the second phase. Carrington has said agreement must first be reached on the constitution.

The threat by former prime minister Ian Smith in Salisbury yesterday to attempt to block Muzorewa's conditional acceptance of the constitution are adding to the atmosphere of brinksmanship at the conference.

Nevertheless, many observers feel that given the military and political situation in Zimbabwe-Rhodesia, such crises are to be expected at the conference and that Carrington's tactics of bringing the constitutional issues to a head will succeed.

The theory is that both the Front and Smith, the leader of the 230,000 white minority, are engaged in confrontation negotiations with the British to try to get the most for making eventual concessions.

Mugabe and Nkomo are subject to pressure from the neighboring front-line African states that support them but are hurting from reprisals in the escalating war.

The guerrillas are more interested in the transitional arrangement that will play a key role in determining who takes power than in the constitution. They fear, however, that if the conference eventually breaks down over the transitional arrangements after reaching agreement on a constitution, Britain could still go ahead with Muzorewa to grant recognition and lift economic sanctions.

Thus, they must seek to hedge any eventual agreement on the constitution with as many conditions as possible and show that they gave in only under protest to allow the conference to make progress.

Any bilateral deal between Britain and Muzorewa would require the British to gain international support and that in turn would depend upon what party is blamed for an eventual failure of the conference.

Britain called the conference last month after agreeing in August on the basic elements of a settlement plan at the Commonwealth summit in Lusaka, Zambia. British officials have said frequently during the conference that their proposed constitution follows along the lines of the Lusaka agreement.

For Carrington, the problem is complicated by domestic political considerations since the deadlock comes on the eve of the Conservative Party conference, where right wingers will press the government to lift sanctions.

Carrington must decide Tuesday whether the new Front proposals provide enough room for further useful negotiations.

The key problems raised by the Front were familiar ones that have been the subject of negotiating since the conference started Sept. 10.

They include objections to British proposals that:

Entrench in the constitution for a limited period white land rights and representation in parliament;

Provide citizenship to persons who entered Rhodesia after Smith's illegal declaration of independence in 1965;

Make it difficult for heads of various governmental commissions to be blacks;

Retain pension eligibility for all officials of the illegal government.

The Front did give in on its demand for an all-powerful executive president and instead went along with a British plan for a basically figurehead president and a prime minister as head of government.

Both Mugabe and Nkomo were critical of what they called Britain's "attitude, take it or leave it" and said the British draft presented last week was mainly "a restatement of exactly the same positions as were outlined" weeks before. They accused Britain of not negotiating but "merely going through the motions."

Mugabe described as "stupid" the need for unanimity in the national assembly to amend the entrenched clauses, adding, "no country in the world has that kind of restriction on its parliament."