The Soviet Union today began the long process of officially placing its new proposals before U.S. negotiators at the arms control talks here, according to Soviet and American sources.

Soviet Ambassador Viktor Karpov, at a 40-minute meeting this afternoon at U.S. delegation headquarters, spent 30 minutes reading a paper on the Soviet position and handed a copy of it to the chief American negotiator, Ambassador Max Kampelman.

The substance of Karpov's statement was not made public, but the Soviet diplomat told reporters before the meeting that he was "going to start introducing our proposals today" and he described them as "drastic solutions" to the superpower stalemate over reducing nuclear arsenals.

When he emerged from the brief session, Karpov said he would continue his presentation Tuesday at a specially arranged morning meeting at the Soviet delegation headquarters. Kampelman said the American delegation had "a reaction of interest" to the Soviet opening statements and that "we will study those proposals."

Specifics of the actual Soviet proposal were not made public, and there was some speculation, both here and in other capitals, that the full details may not be available until Soviet leader Mikhail Gorbachev arrives in Paris for a visit on Wednesday.

Kampelman told reporters before today's session that "we want to get a serious offer and hope we get it." He refused to speculate whether some sort of agreement could be worked out before the Nov. 19-20 summit meeting between President Reagan and Gorbachev. But other officials, both here and in Washington, have noted the complexity of the issues and said any significant advances within that short period of time are doubtful.

Before today's meeting, U.S. officials said they were uncertain what to expect from the Soviets. The session originally was requested on Sept. 19, the first day of the third round of the Geneva talks.

At that time, sources said, the U.S. delegation was told two special sessions were needed to present the proposals that Soviet Foreign Minister Eduard Shevardnadze would deliver to Reagan. Since that time, the delegation has been waiting to see what form the proposals would take.

At the Friday meeting in Washington, Shevardnadze said one part of the Soviet plan would be for both sides to reduce their nuclear arsenals by 50 percent, according to U.S. sources.

The Soviet plan also reportedly would limit how much of each country's remaining strategic force could be maintained in one type of delivery system, such as ground-based intercontinental missiles.

A 60 percent limit was mentioned by Soviet sources earlier this month in Washington.

In addition, the Soviets are expected to call for constraint on the testing associated with the president's strategic defense initiative, the so-called Star Wars research program.

Shevardnadze told Reagan that the Soviet Union wants the United States to abandon that program, which will explore whether a futuristic nationwide missile defense is feasible.

It is now expected that after Karpov's broad presentation at a meeting of all the negotiators, the exact details will be discussed in the separate panels dealing with strategic weapons, intermediate-range systems and space weapons.

In the previous two rounds of Geneva talks, the Soviet delegation is not believed to have produced any concrete proposals in any of the three negotiating forums.

In the second round, however, the Soviets informally outlined what appears to be their new approach.

A major question on the part of American officials is how to determine exactly what weapons are to be included in the 50 percent reduction.

By including U.S. nuclear bombs and even the American Pershing and cruise missiles based in Europe, the Soviets could create major obstacles to any agreement and could avoid the need to reduce their largest ICBMs, which is the target of U.S. negotiators.

The first hint of how that problem may be handled could come as early as Wednesday, when the strategic panel is scheduled to meet.