WARSAW, Poland (AP) _ Authorities outlawed an independent students union at Warsaw University on the grounds that it threatened to stir unrest and engage in anti-state activities, organizers of the group said Wednesday.

Members of the organizing committee of the Warsaw University Independent Students Union, known by its Polish initials NZS, said they would defy the ban and organize a rally next week to protest delays in dormitory renovations.

The ban, dated Monday, was signed by the city's socio-administrative department, which has the power to decide the legality of organizations established by citizens.

Student union activists announced at a March 8 campus rally, attended by about 2,000 students, that they were giving up underground activities to work openly for more academic freedoms and to protect students' material interests.

The banning order said the program of the NZS chapter shows the student union's ''intention is to undertake activities which could cause a threat to the peace and public order.''

A copy of the order was made available to Western reporters Wednesday.

Student activists have campaigned at several major universities this year for communist authorities to legalize the Independent Students Union, which was officially registered in February 1981 but outlawed a year later after the martial law crackdown on the Solidarity free trade union movement.

About 1,500 Warsaw University students have joined the Warsaw University NZS chapter since March 8, said Andrzej Papierz, a history student and union organizer.

Also on Wednesday, a Gdansk misdemeanor court meted out fines ranging from 30,000 to 50,000 zlotys ($76 to $128) to 11 opposition activists who were detained Monday at an anti-government happening marking the first day of spring, said Maciej Jankowski, a Gdansk opposition spokesman.

One of those fined, Cezary Waluszko, said he would file a complaint charging that police severely beat him after he was detained at the happening in which a doll with dark glasses like those worn by Polish leader Gen. Wojciech Jaruzelski was thrown into the Baltic, said Jankowski.

In Wroclaw, misdemeanor courts fined two people 50,000 zlotys each for taking part in illegal political protests, said Barbara Labuda, a Wroclaw opposition spokeswoman.