More than 100 Soviet tourists were killed while on a pleasure cruise down the Volga River when their ship rammed a railway bridge near the city of Ulyanovsk, a spokesman for the Soviet tourist organization Intourist said today.

The passengers were watching a movie on the upper deck of the ship late Sunday night when the accident occurred about 430 miles east of here, he said. The entire upper deck was ripped away in the crash.

The spokesman said there were no foreigners among the 300 passengers aboard the Alexander Suvorov.

It was the worst reported river disaster in the country in several decades. Officials here provided no details except that the accident occurred in a channel where the river is about three miles wide. The Volga near Ulyanovsk is more than 25 miles wide except for the channel spanned by the railway bridge.

While announcing the disaster last night, Soviet authorities did not report the number of casualties or the cause of the accident.

There was speculation among Western observers that the number of dead may be greater than 100 since the official accounts did not mention aid being provided to injured passengers.

A high-level investigating commission was formed that included Politburo member Gaidar Aliyev, the first deputy premier, and alternate Politburo member Mikhail Solomentsev, the premier of the Russian Republic. The high level of the commission of inquiry may indicate that the disaster was on a large scale.

The last reported river incident occurred in 1977 when an unspecified number of persons were killed in a collision involving a pleasure cruiser and a tanker near the city of Gorki, also on the Volga. Unconfirmed reports of another disaster on the Volga a couple of years earlier had several scores of persons killed following an explosion of a boiler aboard a river cruise vessel.

The Russians normally do not publicize air or naval disasters unless foreign passengers are involved. The Intourist spokesman said that he disclosed information today so he could assert that no foreigners were involved in the incident.