President Reagan personally overruled objections from top Pentagon officials when he announced long-range plans this week to study a futuristic defense system that could destroy Soviet intercontinental ballistic missiles in flight.

"The quicker we start, the better," he said yesterday.

Senior administration officials said the president insisted on making the announcement in his address Wednesday night, even though some officials questioned whether the timing was right and whether Reagan should have brought the issue up at all. "I'd put it out now because, what better time?" the president said yesterday in a 15-minute question-and-answer session with reporters. "I've been having this idea and it's been kicking around in my mind for some time here recently. And constantly I have thought about the fact that the nuclear missile seems to be one of the only major weapons systems in history that has never produced or brought about a defense against itself . . . . "

He added, "And since we don't know how long it will take or if--or ever, that we have to start--the quicker we start, the better."

Administration sources said that two Pentagon officials, Undersecretary for Policy Fred C. Ikle and Assistant Secretary for International Security Policy Richard Perle, had questioned whether Reagan should even raise the issue in his Wednesday night defense speech.

The sources said Ikle, while supporting the general idea of a defensive system, was doubtful about the timing and format of Reagan's proposal. Perle, who led the internal opposition, worried that it would raise concern that the United States was about to adopt an anti-ballistic missile system and was drifting away from the NATO alliance, the sources said.

The idea first came up the week of Feb. 7 in a discussion Reagan had with the Joint Chiefs of Staff, the officials said.

"When they first discussed it, the president immediately captured the idea and asked for a decision" on a closely held basis, said one informed administration official.

Before and during the 1980 campaign, Reagan expressed interest in a high-technology solution to the "interminable" nuclear arms race, the official said. Reagan asked Ikle, among others, about it during the presidential campaign.

Drafted by the National Security Council and the Joint Chiefs of Staff, the speech text dealing with this high-technology proposal was circulated to other Defense and State department officials only last week, the sources said. It then drew objections from Perle and others, which had the effect of watering down the text, making it less specific, officials said.

But Tuesday evening, they added, Reagan decided he wanted to press ahead in this address rather than leave it for two other planned speeches on arms control and the MX missile. Reagan then rewrote the section of the address dealing with the missile defense system, incorporating some of the objections and making the speech more general in nature, the officials said.

George A. Keyworth, the president's science adviser, who favored inclusion of the futuristic system in the speech as did national security affairs adviser William P. Clark, said yesterday it was a "top down" decision coming from the president, rather than being sent up by administration officials.

Most officials in the White House West Wing were unaware of it until the last minute, sources said. Some have since expressed concern that the high-technology defense system has obscured the larger point Reagan wanted to make in support of his planned rearmament.

Yesterday, Reagan signed a directive giving Clark responsibility for the new effort.

Officials have been vague about the cost, but Keyworth said yesterday that the administration is talking about something at least matching what he said is a $2 billion Soviet effort, about twice the current U.S. spending level. Keyworth also said he expects that a new office will be established within a few months to coordinate the effort, which is now scattered among various agencies.

Although much of the speculation about such a defense system has centered on satellites, Keyworth said yesterday that it is more likely to emerge in the form of land-based laser systems. At the urging of Ikle and others, Reagan stopped short of outlining a more ambitious defense system aimed at Soviet bombers and cruise missiles as well, administration sources said.

Reagan said yesterday that he finds it "inconceivable" that "the great nations of the world will sit here, like people facing themselves across a table, each with a cocked gun, and no one knowing whether someone might tighten their finger on the trigger."

The president said he would not violate the anti-ballistic missile treaty with the Soviets, which just underwent a five-year review. The treaty, he added, bars deployment of, but not research on, defensive weapons.

Reagan also defended his nominee to head the Arms Control and Disarmament Agency, Kenneth L. Adelman, following charges from Senate Democrats that Adelman misled the Senate Foreign Relations Committee in earlier testimony. "You bet, I am sticking by Mr. Adelman," Reagan said.