A group of interned activists of the independent trade union Solidarity have described an offer by Poland's martial-law authorities to allow them to emigrate as a violation of international law, according to a clandestine union bulletin circulating here.

The government's new travel regulations, allowing internees to apply for passports to leave the country with their families, came into force today. But a recent visitor to several internment camps reported that only a small minority of detainees had indicated any interest in emigrating.

The underground bulletin, issued by the suspended union's Warsaw branch, said officials of the security forces had been conducting conversations with many detainees encouraging them to emigrate. The bulletin said that, if they did not accept the offer, they could expect prolonged periods of detention or service in the Army.

Several Western governments, including the Reagan administration, have said they will not accept Polish citizens forced to leave the country.

Official Polish spokesmen, however, have vigorously denied that anyone is being forced to emigrate. They say the offer of passports is a humanitarian gesture designed to solve the problem of what to do with unrepentent "enemies of socialism."

The statement issued in the name of internees at the Jaworzo camp in northwest Poland, where several leading intellectuals are held, said it was not difficult to see "elements of blackmail" in the offer.

"The choice we are all facing boils down to the following: either deprivation of freedom without time limit or lifelong exile," the statement said.

It added: "In accordance with international pacts on human rights, we are in favor of every man's right to settle in the country of his choice, no matter what his citizenship. But forcing people deprived of their freedom to leave the country . . . must be judged a violation of these pacts."

Some former internees have already applied to Western embassies in Warsaw for visas, but have received little encouragement.

Estimates vary about the number of internees likely to accept the government's offer. The best guess is that most of those who decide to leave will be from the union's middle and lower echelons, and few, if any, of Solidarity's best-known leaders will apply to emigrate.

A visitor to an internment camp in southern Poland reported that the reaction among internees there was: "Let the security police emigrate themselves. We are citizens of this country and want to stay here."

The government has allowed several independent visits and inspections of internment camps which are now officially said to have shrunk by half, to 24. The reports based on these visits suggest that conditions in which the detainees are being held vary widely--from relatively comfortable camps, such as Jaworzo, to bad in former Nazi prisons, such as Kamienna Gora in the southwest.

A recent visitor named camps in Rzeszow, Katowice, and Wroclaw provinces--all in southern Poland--as among the harshest for Solidarity internees. He estimated that, in these camps, one in 10 inmates had been maltreated.

Asked to cite examples of beatings, he mentioned the case of a former Solidarity leader at the Katowice steelworks, Jan Domagala, who was detained Jan. 7 and held for three weeks in a police station. As a result of the treatment he received there, he had to spend six weeks in a hospital before being transferred to a prison camp at Jastrzebie near the Czechoslovak border.

Domagala achieved fame last year after he sued a hard-line member of the Communist Party's ruling Politburo, Albin Siwak, for libel. Siwak alleged that he had been found guilty by the courts of a series of crimes, including rape and bigamy.