President Reagan will make a "detailed and substantive" response to the Soviet proposal for a 50 percent reduction in strategic nuclear weapons, probably in advance of his summit meeting with Soviet leader Mikhail Gorbachev, administration sources said yesterday.

These sources said the timing and substance of the U.S. response depended on presidential decisions that will be made during the next two weeks. The Soviet proposal has been fully analyzed, but differences on the reply remain between the State and Defense departments, the sources said.

Administration officials said there is no "artificial deadline" for a decision, but said Reagan is likely to discuss the major features of the U.S. response in a nationally televised speech tentatively scheduled the week before the Nov. 19-20 summit in Geneva.

The speech will center on arms control and present Reagan's "vision of U.S.-Soviet relations," according to a senior official.

On Nov. 21 the president will stop off in Brussels to brief Allied leaders on the summit and address Congress in a joint session that night if he receives an invitation to do so, officials said.

Reagan is scheduled to meet today with his national security advisers and preside at a National Security Council meeting on Friday. The president also will confer with Secretary of State George P. Shultz before Shultz departs Saturday for Moscow, where he will discuss the summit with Gorbachev.

"There's a possibility that Shultz will be taking presidential decisions with him to Moscow, but that isn't certain yet," said a senior official.

Some officials said that differences, which they did not specify, remain on the substance of the planned U.S. reply between Shultz and national security affairs adviser Robert C. McFarlane on the one hand and Defense Secretary Caspar W. Weinberger and his arms control specialist, Richard N. Perle, on the other.

Weinberger and Perle are attending a meeting of a NATO nuclear planning group in Brussels and will not return until Thursday.

The Soviet proposal, presented to U.S. negotiators at the strategic arms talks in Geneva last month, calls for a 50 percent cut in strategic offensive arms of both superpowers in return for an end to development and testing of a U.S. space-based missile defense system.

U.S. officials have said the cuts are inequitable because they would not severely restrict the huge Soviet land-based missiles, but Reagan has said there are "seeds we should nurture" in the Soviet offer. Gorbachev's surprise invitation to Shultz last week fueled speculation that the Soviets look forward to a productive summit, but U.S. officials tried yesterday to lower any expectations that either the Shultz-Gorbachev meeting or the summit itself would lead to a new arms control agreement.

"There's a snowball of press speculation as to what we're going to do in regard to a communique or a statement of principles [at the summit], all of which appears to be erroneous, with very few exceptions," White House spokesman Larry Speakes said. "We don't have any agreement with the Soviets to develop a communique, or a statement of principles. We do not oppose one, but we're not seeking one, and we certainly don't judge the success of the meeting with Mr. Gorbachev as to whether or not we have a communique . . . . If one is developed, so be it." Without being asked an additional question, Speakes then launched into a critique of press coverage of the arms control issue.

"There is a second drumbeat in the press that there is disarray, a crippling disarray in the administration, which is allowing us, is forcing us, to delay in replying to the Soviet offer," he said. "Not so . . . . Our proposal was put on the table last spring. The Soviets spent months reviewing it before they came up with a counterproposal. We now have that proposal before us, which we've analyzed in less than a month, and it has not been determined as to whether we would respond to it before, during or after the meeting with Mr. Gorbachev."

Speakes also said there was "a third area of misunderstanding in the press" that the United States had attempted to shift the focus from arms control to other issues.

In a speech to the United Nations last Thursday, the president emphasized Soviet activity in regional disputes throughout the world, especially in Afghanistan. U.S. officials freely acknowledged that the president's emphasis was deliberate because the Soviets had scored public relations successes with their arms control proposal but were uncomfortable when the spotlight was on Afghanistan.

U.S. officials relate the two issues, as Speakes did yesterday, by saying that the SALT II agreement failed because President Carter had to withdraw it after the Soviet invasion of Afghanistan in 1979.

But when pressed as to whether the United States was insisting on a change of Soviet conduct in Afghanistan before there could be progress on arms control, Speakes replied with the standard administration position that there are "no preconditions" for an arms control agreement.

Speakes' criticisms prompted one wire service correspondent to ask him, "Larry, do you wish that you had one state-controlled wire service, one state-controlled network and one state-controlled daily newspaper?"

"No, I just simply wish that I had many voices that spoke with accuracy instead of none who spoke with accuracy," Speakes replied.