MOSCOW, SEPT. 29 -- Tens of thousands of people in Ust Kamenogorsk in Soviet Kazakhstan held angry demonstrations today to protest an explosion two weeks ago at a nearby nuclear fuel plant that officials said could endanger the health of more than 100,000 people.

Kazakhstan's president, Nursultan Nazarbayev, sent a telegram to Prime Minister Nikolai Ryzhkov saying the Sept. 12 accident at the Ulbinsky metallurgical works near the Chinese border spewed dangerous gases over parts of the city and asked that the eastern region of the vast Central Asian republic be declared a disaster area.

"Ust Kamenogorsk and its environs are, I'm afraid, already a badly polluted area, a heavily industrial, metallurgical area, and this just adds to a very bad situation," Nikolai Vorontsov, Soviet minister of environmental protection, said in an interview. "The one good thing you can say is that there has been no raised radiation level detected following the accident in Ust Kamenogorsk. This is bad enough, but it is not a nuclear accident."

Rishat Adamov, chairman of Kazakhstan's Regional Committee on Environmental Protection, said the explosion released toxic beryllium oxide gas into the air and may have contaminated up to 120,000 people. The gas, used as a fuel in the aerospace and nuclear industries, can cause respiratory ailments and lung damage if inhaled.

Following the explosion and fire at the plant, the harmful gas filled the streets of the city for about five hours, dissipating slowly because winds were calm, according to official reports. The Soviet news agency Tass said that later, officials determined there was twice the amount of beryllium "permissible" in the air and in the local water supply.

Sergei Matayev, Kazakhstan correspondent for the weekly newspaper Soyuz, said in a telephone interview that the demonstrators in Ust Kamenogorsk were "furious and scared" and were demanding financial reparations for the disaster. He said the city government had ordered the Ulbinsky plant closed, but officials in Moscow have still not ruled on the matter. "They've been trying for months to shut that plant down. It looks like they may finally succeed," Matayev said.

Adamov said the Ulbinsky metallurgical works was the only plant producing uranium and beryllium fuel in the Soviet Union, but that could not be confirmed.

To protect children, workers have been hosing down schools and streets and spreading sand on playgrounds.

Since the Chernobyl nuclear accident in 1986, the Soviet press has been quicker to report information about various disasters and conflicts in the country. But information on the explosion in Ust Kamenogorsk has come slowly.