President Reagan, expressing undiminished confidence that his economic program will work without harming the nation's poor, yesterday renewed his pledge to veto any spending bill that would "bust the budget."

The president, speaking at a White House news conference as an array of federal aid programs went out of existence with the beginning of the new fiscal year, said adequate benefits remain in place for those dependent on government assistance.

"What we call the safety net is still in place," he said.

"Where the cuts have come is around the periphery," Reagan said, adding that the cuts have been aimed at those who were receiving federal funds while they had private incomes as well.

The president accused unnamed people of making an effort to portray his programs as harmful to poor Americans.

At one point, he appeared to misstate a major impact of his program. Reagan was asked about a single working mother who would have her food stamps and medical benefits cut.

"I don't believe that we are actually doing that," he said, although many working poor will lose medical benefits under his program. Then he conceded that "some may be hurt more than others" and went to say:

"But those people that are totally dependent on government, that is our obligation and nothing is going to happen to them."

The impact on others, Reagan indicated, will be in non-essential areas and, as it would for anyone, "forces us to change some of our plans and reorder our priorities."

Reagan appeared considerably more comfortable handling questions than at his last White House news conference June 16, when he stumbled in answering foreign policy questions.

For the most part, the president stayed away from specifics, turning to a couple of favorite stories and at one point pulling from his pocket and reading a letter from the Securities Industry Association president. It said that contrary to the impression people have drawn from the weak stock markets and continuing high interest rates, Wall Street does support Reagan's policies.

To illustrate how he is trimming, Reagan also displayed the 318 pages of regulations governing the categorical grant programs that he consolidated into block grants and the six pages of regulations now needed for them. He did not mention that the reduction in regulations was accompanied by a 25 percent funding cut.

The long interval since his last news conference was partly because of a feeling among Reagan's advisers that he performs better when giving a speech than when taking questions.

Reagan opened his remarks yesterday by making a joke about the infrequency of his meetings with the press. "Welcome to my first annual press conference," he said. It actually was the fourth White House news conference of his presidency.

Reagan's main message was the one he has been delivering since he took office: his economic program. "We will not be swayed from our plan by every changing current, every passing trend or every short-term fluctuation," the president said of his determination to maintain the course of budget-cutting and tax reductions.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) wryly hailed yesterday as "D-Day for Reaganomics." The Democratic leader said: "It's his baby now. We've given him what he asked for and what he wanted." O'Neill said: "Starting today, the rich get richer."

On several points other than that many working poor will receive reduced benefits, Reagan gave incomplete or misleading answers.

He took credit for a reduction of Treasury bill interest rates from just under 16 percent when he took office to 14 1/2 percent. But the pattern has not been a steady lowering of rates.

The rate was 15.595 percent when Reagan took office, but that was a peak in a month when the average interest rate was 14.7 percent. Rates fell in the spring, hitting a low of 12.5 percent in early April. Then they soared, reaching 16.75 percent at the end of May. After another fall, they rose to 15.8 percent by Aug. 28, before declining again.

Reagan also said: "Now, some of our business taxes are retroactive and go back to Jan. 1 but, of course, they won't be felt until taxpaying time." Taxpaying time comes every three months, so businesses already have felt some impact of tax cuts.

The president contradicted himself when asked whether he would veto a windfall profits tax on natural gas. He said he would have to see a bill before saying whether or not he would veto it.

In July, Reagan wrote to Rep. Glenn L. English (D-Okla.): "I would, with pleasure, veto such a bill."

On another subject, Reagan said, "What has thrown our budget estimates off is the excessive interest we have to pay to borrow the money to pay for the deficit left us by other Congresses."

Interest rates higher than those forecast added $16 billion to his spending estimates, but another $11 billion resulted from other causes.

Only once did Reagan seem ruffled. He was asked about the new White House china that cost almost $1,000 a setting and other symbols of luxury that have led some to call his administration "millionaires on parade."

"Let's set the record straight once and for all because Nancy's taken a bit of a bum rap on that," Reagan said of the china. He said that because of breakage, the White House has to use more than one set of china at state dinners and that the 220 new settings are a private gift at no expense to taxpayers.

On foreign policy questions, Reagan was vague about his thinking on limited nuclear war and whether he is aiming for military supremacy over the Soviet Union.

Asked whether either side could win a nuclear war, he said, "It's very difficult for me to think that there is a winnable nuclear war, but where our great risk falls is that the Soviet Union has made it very plain that among themselves that they believe it is winnable."

On military strength, Reagan said he seeks "whatever is necessary to insure that window of vulnerability I've spoken of has been closed and the risk has been reduced of there being a war at all."

He deplored "groups among our allies and here in America who are increasingly vocal in carrying their own message and it is one of pacifism and neutrality and so forth."

Reagan said: "I think they're very unrealistic and if we listen to them, I think we'd all be in trouble."