A strike by 20,000 steelworkers demanding a 70 percent wage hike entered its fourth day Friday and was supported by at least 4,000 workers in two other cities, dissident sources said.

Meanwhile, university students called for campus sympathy strikes in support of the steelworkers.Leaders of Solidarity and other outlawed groups rallied followers Thursday to back the steelworkers amid signs that authorities were toughening attempts to rein in growing unrest.

Witnesses said Friday detachments of motorized riot police arrived in Warsaw and Krakow.

Authorities called the burgeoning steelworker strike "illegal," saying the strike committee did not notify plant management about it in advance.

About 20,000 workers at the Lenin foundry in the southern city of Nowa Huta stayed off the job Thursday after a breakdown in negotiations over a 70 percent wage hike. Deputy Prime Minister Zbigniew Szalajda was to go to the foundry to try to break the impasse.

At the Satlowa Wola steelworks in southern Poland, 3,000 of the 25,000 workers employed there joined the strike, Polish dissident sources said.

Zbigniew Romaszewski, a Solidarity leader in Warsaw, said about 3,000 workers led by Solidarity activist Wieslaw Wojtas, who was fired for organizing a protest rally last week, marched to the office of the plant director. There they demanded a monthly pay raise of $52 and the reinstatement of Wojtas and another activist, Romaszewski said.

In Bochnia, 20 miles west of Krakow, spokesman for the 2,000 workers at another steel mill said they support the striking steelworkers but gave no indication they would join the strike. Half of the workers turned up on time for the morning shift. The balance of the mill's workers were scheduled to report to work later.

A dissident source also said preprations were underway for some kind of show of support for the strike in the copper center of Lubin, in southwestern Poland.

The activism spread to the campuses as students at universities in Warsaw and Krakow organized support for the strikers.

Polish authorities have faced a growing number of wage disputes throughout the country after economic reforms forced consumer prices to rise by 42 percent in the first quarter of the year.