Poland's military authorities tonight announced the arrest of one of the top underground leaders of Solidarity just days before the independent trade union's expected dissolution.

The main evening television news bulletin named the Solidarity official as Wladyslaw Frasyniuk, the 28-year-old leader of the union's Wroclaw region in southwestern Poland. He was charged by a military prosecutor with organizing illegal demonstrations and other protests against martial law.

Frasyniuk managed to escape arrest when martial law was declared last December and went into hiding in the Wroclaw area. He was one of four senior Solidarity officials who formed a provisional coordinating commission last April to run the union's affairs while the rest of its elected leaders were interned.

Frasyniuk's arrest is a major success for the martial law authorities as they prepare for an important session of parliament this weekend that will discuss the dissolution of Solidarity and other trade unions. The arrest could severely undermine protests against a new trade union law which parliament is expected to pass.

Solidarity's Wroclaw region was regarded as one of the most radical and the source of the most determined opposition to military rule.

Solidarity's underground leadership is made up of union officials from Warsaw, Gdansk, Krakow and Wroclaw who hold about one meeting a month. Frasyniuk's arrest will make it more difficult for the remaining underground leaders to meet and could explain why the provisional commission has not yet reacted publicly to the government's proposed trade union law.

The parliament, or Sejm, has been called into session Friday and Saturday to vote on the new law which would dismantle existing unions and set stringent conditions to the formation of new ones. The draft law, copies of which have been leaked to Western journalists here, severely limits the right to strike and effectively excludes Solidarity's former leadership from any role in union affairs.

Friday's session is seen here as a watershed event in Poland's long-running political and economic crisis. Under martial law regulations introduced last December, Solidarity was suspended "temporarily" along with other trade unions and professional associations.

The decision to dissolve the trade unions altogether was apparently taken by Poland's military leader, Gen. Wojciech Jaruzelski, and senior Communist Party officials after nationwide street demonstrations in August and September. It is strongly opposed by the Roman Catholic church, a powerful political force in Poland, which has called for Solidarity's reinstatement and the release of hundreds of political prisoners still held in internment camps.

The primate of Poland, Archbishop Josef Glemp, has canceled an important visit to the Vatican and Washington to keep in touch with developments. Last week he was quoted as telling a group of editorial writers from the United States that "major disturbances" could be expected if the authorities outlawed Solidarity.

Glemp's assessment is shared by many other observers including some members of the Communist Party establishment who are worried about the prospect of mass protests.

In a statement last April, Solidarity's underground leadership said it would not hesitate to call a general strike if the union was ever disbanded. The statement said that workers would be asked to "actively defend" their factories if attacked by riot police.

More recent underground statements have been less explicit, and there is now considerable doubt over whether Solidarity has the organizational ability to stage an effective strike. The union's communications system is poor and the factories have been heavily infiltrated by security police.

The alternative to strikes is street demonstrations--either spontaneous or coordinated by the underground. The government, however, seems confident that it has learned how to deal with such protests through the massive deployment of riot police supported by Army units.

The authorities are presenting the trade union bill as a chance to start from scratch after the upheavals of the past two years.

Passage of the bill through the Sejm is regarded as a foregone conclusion, since the Communist Party and its political allies hold an overwhelming majority in the legislature.