Rhodesian Prime Minister Ian Smith all but formally rejected the latest Anglo-American proposals for a peaceful settlement of the Rhodesia dispute today, calling some of their key provisions "crazy" and "insane" and the whole plan "ill-conceived."

"I think it is a very cunning scheme to insure that the Patriotic Front will be the next government of Rhodesia," he told a news conference here. "I have no doubt in my mind what the ultimate objective is."

The Patriotic Front is the main black nationalist group fighting to overthrow the white-minority government in Rhodesia.

Nonetheless, the Rhodesian leader said he was not rejecting the plan out-of-hand and would await a further analysis by his legal experts of the 24-page British document before giving a "considered reply."

He said there is still a lot of uncertainty about the meaning of various parts of the plan and that after talking with British Foreign Secretary David Owen and U.S. Ambassador Andrew Young for more than three hours yesterday he was not clear what is still open for negotiation and what is not. Whether he would agree to discuss the plan he said, depends on how flexible its terms prove to be.

The British proposals, worked out with and fully backed by the United States, call for the "surrender" of power by Smith to Britain and Rhodesia's return to its pre-1965 status as a British colony so a black-majority government can be established here peacefull by the end of 1978 with the help of a United Nations peace-keeping force.

While Smith left his final answer for later, he made it clear once again that he intends to go ahead immediately with his own plan for an internal settlement with more moderate nationalist leaders inside the country.

He rejected the contention of both Owen and Young yesterday that this would touch off far more violence and bloodshed than before, with black fighting black, and he also laughed at the U.S. spokesman in Washington who described the proposals as the "last best chance" for the 270,000 whites in Rhodesia.

"I think I'm not exaggerating when I say that in the last 10 years, I've had 10 last chances and we are still going," he said.

British and American sources said earlier that they fully expected Smith to dismiss the proposals and that they planned to take them to the United Nations for endorsement while awaiting changes in Rhodesia that might make them more acceptable later.

The first reaction from the black nationalist patriotic Front to the proposals was critical but not a rejection. Joseph Msika a Front spokesman in Lusaka, said they were "deliberately designed and calculated to protect the racist white-minority settlers" during the transition period and after the establishment of black-majority rule.

He said last night that the Front demanded the "immediate surrender of power" by the Smith government. Otherwise, he said, it would intensify the armed struggle.

The five "front-line" states supporting the front have not commented on the proposals yet. But they seem to meet their two key demands - the removal of Smith and the virtual dismantling of the white-controlled army in Rhodesia.

Smith made it clear throughout his 45-minute press conference that he rejected most of the main features of the Anglo-American peace plan. He said it was a "crazy suggestion" to call for dissolving the white Rhodesian government before it was known precisely what the new constitution was.

The proposal to dismantle the white-controlled Rhodesian army and establish an international force in its place, he said, is "almost insane." Only if the U.N. force were "under the control of our command," Smith said would he be prepared to consider the proposal. "I think it would be acceptable, in other words on our conditions," he said.

He also said that the idea of holding elections for a black-majority government on the basis of one-man one-vote "would not be in the best interest of Rhodesia" and that even the British and American envoys had not made it clear what they meant by universal suffrage. He did not, however, specifically rule it out.

In addition, he termed the proposed British resident commissioner who would govern Rhodesia during an interim period a "complete dictator" and said that the plan's guarantees for whites are not adequate.

"It seems to me that the plan is not only ill-conceived, it is rushed," he said, adding that the use of such terms as "surrender" by the British was counterproductive to their own efforts to get him to accept their constitutional settlement.

"I detected more than I have ever before on the part of Dr. Owen an attempt to exact retribution on the Rhodesians for what took place in 1965. It seems to me . . . that they are seeking revenge," he said.

It was in 1965 that the whites of Rhodesia unilaterally declared their independence of Britain, with Smith leading the rebellion since the begining.

In contrast to his biting remarks about the British, Smith praised the Americans and said he regrets that the two countries have not had closer relations. "I believe they are adopting a more pragmatic attitude than the British," he said. "We did not detect yesterday from the Amerficans the kind of vindictiveness that we detected among the British.

"I would have thought we might have had a better chance of coming to an agreement had we been dealing with the Americans because of their straightforward, practical approach to problems" he added.

But he said hid he would not indulge in "wishful thingking" about the possible greater flexibility of the U.S. government or appeal to President Carter over the heads of the British.