Management Thursday said a strike at one of Poland's largest steel mills was illegal and threatened to call in riot police to end the three-day walkout, strike leaders said. Organizers said 16,000 workers, or half the work force, were on strike at the Lenin steel works outside Krakow in Poland's largest work stoppage since martial law stifled Solidarity 61/2 years ago. The government has raised prices 42 percent so far this year as part of a program to make the economy more market-oriented, and there is widespread discontent that wages are lagging behind inflation.