The future of the Solidarity union largely depends on the outcome of mass demonstrations planned for next week in defiance of martial law, according to an underground leader of the union.

In a statement published in a clandestine Solidarity bulletin that reached Western correspondents here today, Zbigniew Bujak acknowledged that the decision to organize street rallies throughout Poland carried considerable risks and could result in bloodshed. But he defended it as being preferable to uncoordinated local protests and necessary to force the government to negotiate with Solidarity.

The head of Solidarity's once powerful Warsaw chapter, Bujak, 28, managed to escape arrest during the military takeover in December and has been in hiding since. He is regarded as the most influential figure in the union's five-man underground leadership, which issued the call for demonstrations next Tuesday to mark the second anniversary of the Gdansk agreement that recognized the right of workers to form free trade unions.

His statement in the latest issue of Solidarity's Warsaw weekly, Tygodnik Mazowsze, provides an insight into the thinking of the underground as it prepares for perhaps its most important test yet. The Communist authorities have vowed to prevent the demonstrations from taking place, and the Roman Catholic Church has advised Poles to keep off the streets.

A measure of the seriousness with which the government views the threat of nationwide demonstrations came in a toughly worded speech by the Communist Party secretary responsible for organizational matters, Kazimierz Barcikowski, to shipyard workers in Szczecin, one of the possible trouble spots. He charged the protests were designed to pave the way for an all-out general strike which, if successful, would be followed by an armed insurrection against the Communist regime.

Further evidence of the government's concern came Thursday when Polish television said officials moved forcefully against demonstrators in Lodz and arrested 108 persons following a clash with police, The Associated Press reported. Seven policemen were reported injured.

There were sketchy reports of violence in Gdansk and Szczecin.

Polish television broadcast footage of the summary trials of those arrested in Lodz in an apparent attempt to frighten other would-be demonstrators. It said all 108 received stiff fines.

Explaining why Solidarity's provisional leadership had decided to call for demonstrations, Bujak said it had been seeking the most effective and visible form of protest. The government had been able to hush up the scale of token strikes held inside factories but was much more afraid of public protests in which the demonstrators outnumbered the security forces.

The article, entitled "Defense of the Union," made clear that Solidarity is planning a different form of demonstration from previous rallies, which were largely spontaneous. Bujak said much would depend on "organized groups of demonstrators" whose task it would be to prevent attempts by the security forces to disperse the crowds.

The new Solidarity tactics, according to Bujak, are based on lessons learned from rioting in Wroclaw on June 13. On that occasion, he said, the crowds lost their sense of fear and managed to throw the elite riot police known as Zomo onto the defensive.

"In such an event, the Zomo have a very difficult choice: whether or not to shoot," he said. "As the people's power, they are afraid to shoot--but it's always possible that they will."

He added: "The course of the marches on the August anniversary will, to a significant extent, decide the strategy which we adopt in the forthcoming period. If it turns out that the people are not afraid and organize themselves so as to prevent any attempts to break up the demonstrations, that will mean the continuation of a radical policy of pressure on the authorities. In a relatively short time, the government will be forced to talks with the church and the union.

"If, on the other hand, the authorities succeed in dealing with the demonstrations, that would mean that we are not capable of undertaking any radical form of resistance and that we must abandon mass actions. Then all that would remain for us would be long-term resistance."

Bujak said that a failure to undertake protests in August would convince the authorities of Solidarity's weakness and remove all obstacles to its complete delegalization. The union, which is at present only suspended, would be forced underground for good, he said.

It is impossible at present to predict how widespread next Tuesday's protests are likely to be. In Warsaw, underground organizers have called on workers to gather in four central squares at 4 p.m. But the security forces seem equally determined to block access to the city center.

In his statement, Bujak said underground union representatives in all major factories supported the idea of demonstrations. Many people, however, may be deterred from participating by the government's intensive propaganda campaign warning of possible bloodshed.