Yugoslavia has expelled two American human rights activists who were here to influence the European Security Conference, the U.S. embassy confirmed today.

Since the conference began two weeks ago, at least 20 foreigners have been ordered out on the ground that they sought to disturb the work of the delegates.

The U.S. embassy identified the two Americans, expelled yesterday, as Andrew Fedynsky and Adam Misztal of the Washington-based "Helsinki Gurantees for the Ukraine Committee." They were put on a plane to Vienna after being prevented from holding a press conference at their hotel.

While entry in Yugoslavia is the easiest in Eastern Europe, security is much tighter around the conference center here. The police seem to hear about any planned press conference.

News that women campaigners for Sovie Jewry were in town large numbers of journalists and plainclothesmen on the eve of the opening ceremony. The women themselves were rumored to be hiding in he toilet waiting to be interviewed.

Fedynsky and Misztal, less secretive, pinned up a notice at the conference center announcing their press conference in the luxury Jugoslavija Hotel. Shortly before the planned time, journalists saw a struggling man being bundled into a blue car and driven off at high speed.

Hotel receptionists said the two campaigners had just checked out.

The Yugoslav government, to justify the expulsions, notes that the protestors enter on tourist passes, whch do not allow them to engage in such political activity as holding press conferences or handling out leaflets. Officials add that they have a special responsibility to prevent hostile acts against any of the 35 participating nations.

However, while Jewish activists were being expelled, a "disarmament relay team" that had arrived from Helsinki received assistance to hold press conference and hand out leaflets.

The relay team, which included buddhinst, monks, even received a message of support from Yugoslav Foreign Minister Milos Minic. Yet they, like nearly everybody else who enters Yugoslavia for a short period, had come in on tourist passes.

Whatever tactics Yugoslavia employs against the human-rights campaigners, it seems unlikely that they will stop coming to Belgrade. The campaigners themselves predict a much larger number of lobbyists for the main conference in the fall and a senior U.S. delegate said that the present trickle of campaigners could turn into a flood in October.

It is a prospect that clearly concerns the Yugoslav organizers.