CHICAGO (AP) _ The city that boasts the world's largest Polish population after Warsaw on Thursday prepared a joyous welcome for Solidarity leader Lech Walesa.

''Next to the pope's visit here, it's the biggest thing that's happened,'' said Anthony Piwowarczyk, vice president of the Polish National Alliance.

''There's no question about it,'' said Roman Pucinski, a city alderman and president of the local Polish American Congress. ''The two top figures of the world today, the pope and Walesa, are both Polish.''

Signs depicting Walesa were taped to store windows up and down a section of Milwaukee Avenue, center of the city's Polish community. ''Solidarnosc: A labor leader who is changing the world,'' read one, urging attendance at a rally Saturday in Walesa's honor. Another read simply: ''The Man.''

As many as 50,000 people are expected at the rally, including thousands of children from Polish ''Saturday Schools,'' schools in Polish language, history and culture. It is Walesa's only public appearance in Chicago, with private meetings occupying most of the rest of his one-day visit.

When asked about Walesa, Wojciech Cioromski immediately pulled two invitations from his breast pocket - one to a Saturday breakfast and the second to a lunch honoring the labor leader.

''Absolutely. They are waiting for Walesa,'' said Cioromski of Chicago Poles as he drank coffee and chain-smoked European cigarettes at the Orbit Restaurant, across the street from Klub Ameryka.

''Everybody wants to see him - but he spends minutes here,'' he said.

Walesa will be in Chicago slightly more than 24 hours after he arrives Friday, and the demands for his time have been monumental.

''They are calling Solidarity headquarters in Gdansk, asking him to attend weddings, birthday parties, anything you can imagine,'' said Pucinski, who met with Walesa while in Poland earlier this month. ''He said to me, 'It would be easier for me to go to the moon than go to Chicago.'''

Some 800,000 Poles live in the Chicago area, more than in any other city but Warsaw, said Piwowarczyk.

While some along Milwaukee Avenue said they were saddened Walesa would spend none of his time in the heart of the Polish community, most said they understood.

''He's not going to be going by in everybody's little delicatessen,'' said Peter Bacik, whose father owns Bacik's Delicatessen. ''But we appreciate what he is doing - we are all for it.''

''It's been something,'' Bacik reflected while standing among customers eyeing rows of hanging sausages. ''They are watching the news, listening to Polish radio stations. Polish people are finally coming out of the dark ages.''