President Carter took the extraordinary step yesterday of vetoing a $36 billion weapons procurement bill that he said would weaken the nation's military preparedness.

Announcing his decision at a nationally televised news conference, the president said he was vetoing the bill because Congress included in it $2 billion for a new nuclear-powered aircraft carrier that is opposed by the administration, diverting the money from more important military projects.

"This is not a question of money," he said. "It is a question of how that money is going to be spent, whether it will be concentrated in the most vital areas of need or diverted to less crucial projects."

Democratic congressional leaders generally predicted the veto would be sustained and some administration officials were even more optimistic. But other administration officials also said that sustaining the veto will be "extremely difficult" and that the venture involves "high risks" politically for Carter.

The veto was Carter's fifth - the four other relatively minor vetoes were sustained - and represents his most serious challenge to Congress in his 19 months as president. Officials who researched the subject said they found the last time a major defense authorization bill was vetoed was during the 1845-1849 administration of President james K. Polk.

Carter acted yesterday amid his continuing slide in the public opinion polls and a growing image of him as weak in dealing with Congress. There have been suggestions in recent days that the president is about to embark on a much tougher approach toward Congress, where many of his major bills remain stalled or have alreay been killed for this session.

But as he has so often in the past. Carter sounded conciliatory in speaking of Congress yesterday and denied that his decision to veto the defense bill was in any way linked to White House efforts to buttress his image as a hard-nosed chief executive.

"I would say that in general the Congress has been very cooperative and very constructive," he said.

But the president added that there are other bills he thinks now he should have vetoed - he cited specifically last year's public works measure that included water projects he opposed - and that he will not hesitate to use the veto weapon in the future.

"I don't have any fear of Congress and I'm sure they have no fear of me," he said.

The key issue in the weapons authorization bill is the inclusion of the $2 billion for a fifth nuclear carrier, long opposed by Carter as too costly and vulnerable to attack. The administration favors the construction of a larger number of less costly, conventionally powered naval warships, and has already promised to include funds for a new conventionally powered carrier in its budget requests for the 1980 fiscal year.

Carter argued that to allow for construction of the nuclear carrier, Congress slashed more vital programs, many of them affecting the ability of the United States to fulfill its commitments as part of the North Atlantic Treaty Organization. He cited cuts of $800 million for Army weapons and equipment, $200 million for Air Force weapons and equipment, $500 million in military operational funds for such things as spare parts and logistical support and a cut that could range up to $500 million for research and development.

"By diverting funds away from more important defense needs in order to build a very expensive nuclear aircraft carrier, the bill would reduce our commitment to NATO, waste the resources available for defense and weaken our nation's military capabilities in the future," the president said.

Administration officials said they would prefer Congress to enact a new authorization bill eliminating the carrier and restoring funds for the programs cited by Carter. But they made it clear they would settle for the mere deletion of the carrier, sending later supplemental budget requests to Congress for the other items.

The veto is bound to complicate further the already clogged congressional calender, delaying action on the defense appropriations bill until the weapons authorization issue is settled.

The House agreed yesterday to consider the veto Sept. 7 after it returns from its Labor Day recess. Underscoring the political importance to Carter of the coming veto fight, the White House immediately put together a task force, headed by Richard Moe, Vice President Mondale's chief of staff, to direct the administration effort. Overriding a presidential veto requires a two-thirds vote by both the House and the Senate.

Carter met yesterday with congressional leaders to discuss the veto and, according to White House officials, received a "very good" response from them.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said he "would presume" the veto would be sustained and that the House leadership "will be doing our part" to support the administration.

However, House and Senate Republicans are expected to be united in voting to override, a factor that House Majority Whip John Brademas (D-Ind.) said will make sustaining the veto "a hard fight."

Rep. Bill Frenzel (R-Minn.), who voted against the nuclear carrier, said he would vote to override the veto. "Some of us who voted against the carrier believe it is a poor reason to veto a defense bill," he said.

Another Republican, Sen. Jake Garn of Utah, said he considered the veto "irresponsible", and added that he will refuse to allow a time limit Carter seeks on debate over the administration's civil service revision bill if the carrier is killed.

"The president needs to learn what linkage is," Garn said. "He has come up here and got the Republicans to bail him out on the Turkish arms embargo and on the (Mideast) plane sales and then he knifes us in the back."

Carter's decision to veto the bill and risk the consequences should a Democratic-controlled Congress override him on such an important issue takes on added significance because of the apparently growing tension between the administration and Capitol Hill. Earlier this week, for example, Agriculture Secretary Bob Bergland told reporters that he plants to take retaliatory measures against congressional Democrats who persist in taking "cheap shots" at the president.

Asked about this at the news conference, Carter denied that was his attitude.

"I have no inclination to do that," he said. "It is not part of my nature and I think it would be counterproductive if I attempted it."

Yesterday's news conference was Carter's 36th as president. All three networks televised it live, but it was carried live in Washington only by WRC-TV (Channel 4). WJLA (Channel 7) and WDVM (Channel 9) carried the congressional testimony of James Earl Ray.