Industrial unrest in Poland spread to the important shipbuilding port of Gdansk today, posing the most serious challenge to the country's communist authorities since a wave of strikes began nearly seven weeks ago.

According to dissident sources, about 15,000 workers occupied the Lenin shipyards in Gdansk and issued a list of sweeping demands, including the disbandment of official trade unions. The Communist Party chief in the region, Tadeusz Fiszbach, was reported to be negotiating with the representatives of the strikers.

The fact that unrest has now spread to the key Baltic ports has grave implications for the Polish leader, Edward Gierek, who is believed to be on holiday in the Soviet Crimea. It was in Godansk, nearly 10 years ago, that workers' riots toppled his predecessor, Wladyslaw Gomulka, and led to Gierek's rise to power.

In December 1970, police fired at workers an attempt to disperse the crowds, killing at least 55 people. This time, there has been no suggestion that the authorities intend to use violence, and intensive negotiations are under way to find a solution.

But even assuming it is settled peacefully, the new strike in Gdansk represents a big challenge to Gierek's prestige and authority. It means that the labor troubles have now spread to every important industrial center in the country with the exception of his own power base of Silesia in the South. t

As the strikes continue -- and there have been more than 150 of them since meat prices were raised July 1 -- they appear to be acquiring greater political content. So far, Polish officials have insisted that the unrest is purely economic.

In addition to the creation of new trade unions, the Gdansk workers are reported to be asking for the publication of their demands in the official press. In a country where the mass media is subject to stringent censorship, this is clearly a sensitive political issue. So, too is the demand that family allowances be raised to the level enjoyed by the police.

The workers also demanded a 20 percent pay raise or about $88, and the cancellation of meat price rises that originally triggered off the strikes.

In a conciliatory gesture, the shipyard management has already agreed to reinstate two workers who had been dismissed after they advocated the formation of free trade unions. They also agreed to the building of a memorial to workers killed in the 1970 riots.

Tonight both Polish state radio and television devoted rare commentaries to the labor unrest. Until now coverage has been very scanty in the official media. Most Poles have learned about the strikes by listening to the Polish-language broadcasts of radio Free Europe.

The radio commentary described some of the workers' grievances as "just" but condemned what it called "work stoppages" -- a euphemism for strikes -- adding that they only added to the country's grave economic problems. It confirmed that the unrest had spread to the Gdansk shipyard without going into detail.

But the commentary said that while some of the workers' demands could be settled by negotiation, others were unacceptable. Observers believed this indicated that the authorities would resist calls such as that for the disbandment of official trade unions or the lifting of censorship.

So far the tactic has been to pay whatever it costs to get the workers back to work but to reject out of hand demands for political reform. At a press conference earlier this week, a senior member of the rolling Politburo, adopt a capitalist-type market economy.

Meanwhile, in Warsaw, a partial striek by transport workers went into its fourth day, disrupting commuter traffic. Some of the workers have returned to work.

Throughout the present unrest, the police have kept in the background. Tonight a procession of some 5,000 people was allowed to march unhindered through central Warsaw after listening to a fiercely nationalistic speech commemorating the death of a Polish hero who opposed Russian rule under the czars.

There was loud applause from the crowd when the speaker stressed Poland's right to exist as an independent nation and attacked the policies of "militarist Russia." The historical analogy needed no explanation to a people that have traditionally been caught between the Russian and German empires.

Such processions, which would be impossible to imagine in most other communist countries, have become commonplace in Poland over the last few months. They underline the fact that Poland is rapidly becoming the communist world's first genuine pluralist society -- whether the authorities like it or not.