A group of detained activists of Poland's suspended independent trade union federation Solidarity has smuggled a message to sympathizers complaining of deteriorating living conditions in their internment camp. At least 80 of them were said to have been on a hunger strike as of late last week.

According to a copy of the message, which came from the Bialoleka juvenile prison in northern Warsaw, where more than 260 detainees are being held, officials of the Polish security services have taken over the running of the camp from the original prison wardens. The detainees said new restrictions had been imposed on their freedom of movement and assembly in the past two weeks.

While most opposition to the martial-law authorities has been peaceful so far, newspapers reported today a small explosion in a telephone booth near the headquarters of the Communist Party Central Committee in Warsaw. The explosion blew out the windows of the booth and wrecked the telephone, but caused no injuries.

Polish officials have said they believe they can contain underground activities by Solidarity activists distributing information. But they have warned of a sharp reaction in the event of any large-scale terrorism.

The message from the internment camp contradicts assertions by Polish officials that conditions in all such camps are good. In a report to parliament last week, the deputy interior minister said rules had been changed recently to allow more freedom to the detainees, including the organization of cultural activities and more frequent visits from outside.

The message smuggled out of Bialoleka, however, says that conditions in the camp are very different. It said the detainees still did not have a right to knives and forks and were obliged to cut their bread with the handles of spoons.

Running and jogging had been banned and visits by families limited to one a month, the message added.

Bialoleka is one of about 50 internment camps established under martial-law regulations Dec. 13. Many of the best known Solidarity activists are believed to be held there, including dissidents Jacek Kuron and Adam Michnik, former press spokesman Janusz Onyszkiewicz, and several members of the union's national commission such as Andrzej Gwiazda and Jan Rulewski.

In their message, the detainees said riot police entered the grounds of the prison Jan. 1 wearing gas masks and carrying clubs. They threatened the internees with beatings unless they stopped singing carols.

Before New Year's Day, conditions at Bialoleka were reported to be relatively good with internees allowed frequent visits and cell doors left open to enable them to meet each other freely. The detainees even set up their own self-government body and elected Bronislaw Geremek, a medieval historian who advised Solidarity leader Lech Walesa, as its chairman.

The latest message, however, said meetings between cell representatives and the camp administration had been ended and the right to participate in "cultural and educational activities" had been scrapped permanently. Attendance at a daily Catholic mass had been limited to groups of 30 detainees at a time, while others were obliged to listen to a relay broadcast in their cells.

The detainees said many of them had gone on hunger strike at the beginning of this month to protest the new restrictions and, as of last week, more than 80 were participating. The hunger strikers were transferred to a separate bungalow on Jan. 8, and all contacts with the rest of the camp were forbidden.

The message said that some hunger strikers, including Geremek, apparently had been transferred from the camp Jan. 9.

Several leading Polish actors, meanwhile, reported that they had been summoned to police headquarters in Warsaw to explain their "demonstration" earlier this month at trials of workers accused of leading strikes to protest martial law. The actors had been attending the trials, which were open to members of the public, but denied taking part in demonstrations.

One well-known actor, Daniel Olbrychski, was questioned for several hours by police yesterday to find out how an appeal by intellectuals to the martial-law chief, Gen. Wojciech Jaruzelski, got out of the country. The appeal, which was signed by Olbrychski and several other prominent cultural figures, warned of possible bloodshed unless martial law were lifted soon.

A government spokesman attempted today to put a good face on the public withdrawal by a leading Solidarity official, Zdzislaw Rozwalak, of a statement supporting the martial-law authorities. The disavowal was made during a trip by foreign correspondents to the western city of Poznan organized by the Foreign Ministry press center.

Questioned about the episode, Jerzy Urban, the government's press spokesman, said the matter was "of no interest" to the authorities. He added that he had heard that Rozwalak believed that his disavowal had been misinterpreted and taken out of context as he was "not used to talking with journalists."

Officials at the press center said today that Rozwalak, the former head of Solidarity's Poznan chapter, had been in touch with them by radio in an attempt to withdraw his withdrawal. It was impossible to contact Rozwalak directly, since telephone communications with Poznan, along with other cities, have been interrupted since martial law began.