American and Soviet negotiators met for more than 2 1/2 hours today in their first major negotiating session on reducing nuclear weapons in Europe. While the two sides are pledged to silence, they were understood to have stated their generally well-known, and widely divergent, opening positions.

The tone of the closed-door meeting was understood to have been serious and frank, as the warm handshakes that ushered in the informal opening of these negotiations yesterday dissolved into the high-stakes business of trying to bridge an enormous difference in outlook about the respective atomic arsenals in Europe.

In the battle for public opinion that went on before these talks began, President Reagan and Soviet President Leonid Brezhnev both made dramatic gestures meant to portray their sincere interest in reducing atomic arms. These were aimed mostly at a West European population that is becoming increasingly nervous about the new round in the arms race that will occur if these negotiations are not successful.

Reagan proposed the so-called zero option, in which the United States would forego the planned deployment of 572 new Pershing II and cruise missiles if the Soviets would dismantle about 600 intermediate-range missiles already in place that can strike West European targets.

Brezhnev previously had talked about a moratorium on missile production and then added sweeteners, such as proposing to make preliminary cuts amounting to perhaps hundreds of missiles, although he did not say which missiles or under what conditions. The Soviets also have talked of sweeping renunciations of all nuclear weapons in Europe.

It is those general positions that were understood to have been put forward today without much further comment.

The Soviets have previously rejected the basic zero option plan, arguing that there is already a rough balance of atomic striking power and that dismantling their existing forces would put them at a disadvantage. The United States has rejected the moratorium idea as something that would leave the Soviet missile edge in place, while the more sweeping renunciation is seen as a way for Moscow to force Washington to put all its jet aircraft based in Europe on the bargaining table as well as British and French nuclear forces.

Although both sides here are pledged to tight secrecy concerning details of the negotiations, one glimpse of the differences did surface publicly when it became clear that the two sides could not yet agree on a name for these talks.

The United States is calling them Intermediate-Nuclear Force negotiations, or INF, the same term used by Reagan in his speech announcing the zero option proposal. This is meant to reflect the American belief that the first thing to be negotiated should be intermediate-range, nuclear-tipped missiles -- able to fly between 1,000 and 3,000 miles -- because they represent the gravest threat to each side's military forces.

A Soviet correspondent here, however, told colleagues that Moscow wants to call the negotiations Talks on the Reduction of Nuclear Arms in Europe, a description that would seem to encompass the U.S. aircraft in Europe and the British and French forces. While the correspondent's remarks are unofficial, American sources here confirm that as yet there is no agreement on what these talks should be called.

The attempt to shroud these talks, at least for the time being, in as much secrecy as possible was obvious today. Reporters were allowed into the room where the first full meeting was held for a few minutes and then ushered out. There were no statements at the conclusion of the meeting by the Soviet delegation and only an American statement reporting the duration of the session.

The first session at which the full delegations of both sides were present was held at the American offices of the U.S. Arms Control and Disarmament Agency here. It is the same room used for discussions with Moscow for the previous Strategic Arms Limitation Talks (SALT) which dealt with the big ocean-spanning missiles and bombers based in the United States and the Soviet Union.

The delegations will meet twice weekly, once at the U.S. building and once at the Soviet compound about a half-mile away. The next meeting is Friday.

Ten representatives of each side sat opposite each other across a long wood conference table. As photographers recorded the gathering, the delegates made small talk.