The following dispatch is based on information arriving from Poland.

Two senior Solidarity leaders who have escaped arrest by Poland's martial-law authorities have urged underground resistance to the government in letters addressed to union supporters, while activists being held in prison have reported that jail conditions have deteriorated.

At the same time, members of the dissolved Independent Students' Association have organized underground opposition, the official Army newspaper Zolnierz Wolnosci reported yesterday, in the first official acknowledgment of student unrest since martial law was imposed Dec. 13.

Classes for schoolchildren throughout the country have resumed, apparently under close government scrutiny. But university classes, which have been closed since the imposition of martial law, will not reopen until the first half of February, according to a statement from the Military Council of National Salvation carried by the official daily Trybuna Ludu.

The government announced Tuesday that the students' association, which had been active in university protests before the imposition of martial law, had been dissolved by the Ministry of Higher Education.

At the time of the military crackdown, the government announced that the student group was suspended, just as Solidarity had been. But officials have said repeatedly that Solidarity will be allowed to continue its trade union activities. By dissolving the students' organization, the government has indicated that it will not be allowed to resume activities.

The Army newspaper account reported that the dissident campaign was centered at the Warsaw Medical Academy, where students have distributed leaflets saying that "the present situation forces us to start underground activity. Students of the Medical Academy should begin passive resistance to all orders. There will be a time for action."

One of the letters from the Solidarity leaders came from Wladysaw Frasyniuk, a member of Solidarity's 18-member presidium and head of the union's Wroclaw regional branch. He wrote, "Remember that our union has not fallen apart from the stomping of Gen. Wojciech Jaruzelski's shoe. It still exists and acts, and its authorities are working because of the will of the overwhelming democratic majority of Polish society."

A warrent for Frasyniuk's arrest has been published in a local Wroclaw newspaper, and Solidarity sources say a 250,000 zloty reward has been offered for his arrest (about $2,900 at the official exchange rate).

The other letter, published in an underground Solidarity bulletin, was signed by Zbigniew Janas, head of the Solidarity branch at the militant Ursus tractor plant near Warsaw.

"In these difficult days, we all must prevent the destruction of Solidarity, which is the only hope of Poles," he was quoted as saying.

Both union leaders cautioned against active resistance to the martial-law authorities, saying too much blood had been spilled already. Janas called on workers to make preparations "in deep conspiracy" for the declaration of a general strike. He urged the formation of secret strike committees and also called for union supporters to circulate as many underground leaflets as possible.

"Remember that the authorities are murderers," Janas worte. "They are indifferent to the number of people they will shoot if it suits their interests."

Frasyniuk said that he was threatened with the death penalty and that under such conditions of repression, the union had to devise new forms of resistance.

Other Solidarity activists and sympathizers, who were detained in the first hours of martial law and are charged with no crimes, are interned in about 50 centers around the country. Several of the union's top leaders are held at Bialoleka, near Warsaw, where according to new reports reaching Western correspondents, conditions have deteriorated.

Informed sources said that some of the staff at the center had been changed because they have been too lenient, allowing prisoners to form a choir, conduct language classes and hold lectures. Those replaced included the head of the prison, the sources said.

A recent underground Solidarity bulletin published a protest by the Bialoleka prisoners, which complained that their treatment was no different from that of ordinary criminals. They have listed eight demands, including the authority to maintain self-goverment, freedom of communication with each other, increased frequency of the monthly visits allowed by relatives, the right to study and the release of the old and sick.

The prisoners said they are planning a hunger strike and will refuse to participate in daily exercise periods, to receive visitors or open their mail.

"Since the prison officials have decided consciously to worsen our conditions, we have stopped talking to them," the protest added.

Meanwhile, the party apparently is purging officials throughout the country, but the extent of the purge is still unknown. Both liberals and hard-liners are known to have been ousted.

Party and government sources have reported that the hard-line Communist leader of the southern industrial region around Katowice has been fired. The controversial official, Andrzej Zabinski, had been blamed for much of the unrest in the region. He was replaced by Zbigniew Mesner, former rector of the Economics Academy of Katowice who was named to the Communist Party Politburo last summer.

Among those reportedly ousted in the purge include several delegates to the extraordinary party congress held last summer, which was among the most democratic ever held in the Soviet Bloc. A Solidarity bulletin said that 15 party members had been arrested in the northern town of Torun, where reform-minded party members organized a controversial seminar last spring aimed at democratizing the party.

The party sources said that the party's relatively liberal and democratic statute has been superseded under martial law by a Politburo instruction allowing the central party organization to make decisions, such as the replacement of local officials, that otherwise would be reserved for elected party bodies.

The government also continued to tighten its grip over other key areas of the country, including the press. Polish journalists, under the process of "ideological verification," are being called in one by one to sit before panels of up to 10 persons to be questioned about their political views. After going through the process, some journalists said it seems clear that the decision as to whether they may continue their careers already has been made.

Thousands of Polish journalists were put under involuntary leave after martial law was imposed, and publication of dozens of periodicals has been suspended.

The government has also made changes in the routine in Warsaw schools. One teacher reported that the faculty and students arrived Monday to resume classes after they were canceled following the military crackdown. They found an Army veteran sitting in the teachers' coat room observing their actions. The teacher said tape recorders and slide projectors had been locked up and the teaching staff's private classroom keys confiscated.

After classes, teachers were called into a meeting with an Army major, a department director from the Education Ministry and a school inspector. The major said there was a need to tighten discipline and emphasized "patriotic content" in teaching programs.