The Patroitic Front guerrillas presented proposals today for a Rhodesian cease-fire that differed widely from a British truce plan as full-scale negotiations began in the final phase of peace talks, now in their 11th week.

The pattern was the same as in the first two phases of the conference where divergences between British and guerrilla positions were bridged by painstaking negotiations by British Foreign Secretary Lord Carrington, chairman of the conference.

Despite the differences, observers generally believe that the participants have come too far to fail now in this ninth attempt to end he 14-year-old Rhodesian conflict. They also say that the Front will concede many of its points within the next week after seeking to gain maximum political capital at the conference.

The key difference between the British and Patriotic Front cease-fire proposals:

Britain wants a cease-fire to be in force in seven to 10 days while the guerrillas say it should be a process of indefinite length with the decision on when it is effective taken by a newly proposed Commonwealth group.

The British want a small force numbering in the hundreds to monitor the truce while the Front proposes a peacekeeping force of thousands with enforcement powers.

Britain wants the police of Prime Minister Abel Muzorewa's outgoing government to be responsible for law and order during the two-month election campaign while the Front wants to include elements of its own forces. e

Britain has proposed moving the guerrilla forces to assembly points while the Front says it will not "surrender its areas under the guise of agreeing to a cease-fire."

The Muzorewa delegation questioned several points of the British plan presented Friday but Salisbury is expected to soon agree and, as in past phases of the conference, Britain will then use this accord to pressure the Front for final agreement.

The guerrillas, led by Robert Mugabe and Joshua Nkomo, are facing conflicting pressures. They need time to infiltrate into Zimbabwe-Rhodesia more forces now based in Zambiz and Mozambique. However Muzorewa, who is already back home campaigning, can prevent their banned parties from carrying out political activity until there is a settlement that will temporarily turn power over to the British.

In continuing efforts to stop guerrilla infiltration, Zimbabwe-Rhodesian forces over the weekend knocked out three bridges in southern Zambia, including a major road bridge on the highway linking Lusaka, the capital, with Victoria Falls.

Last month Salisbury forces cut Zambia's last rail route that bypasses Zimbabwe-Rhodesia by destroying a railway bridge on the line to Dar es Salaam, Tanzania.

In their proposals today the guerrillas also made it clear that they are worried about the possibility of attack inside Zimbabwe-Rhodesia by forces from South Africa, which support Muzorewa.

They called for withdrawal of South African forces from the country and guarantees that there would be no intervention during or after the election.