New Czech Leader Offers to Negotiate With Opposition : East Bloc: Half a million demonstrators turn out in Prague. The city’s Communist Party leadership resigns.

This country’s discredited Communist leadership, scrambling desperately to salvage its grip on power, faced a withering challenge here Saturday from rebels within its own ranks and the most massive show of support yet for the Czechoslovak umbrella opposition group, the Civic Forum.

Karel Urbanek, the 48-year-old one-time railway worker elected as new party chief during a stormy, marathon leadership meeting that ended early Saturday, told a nationwide television audience that the party is willing to “negotiate with everyone who is ready to contribute to the future of Czechoslovakia.”

Urbanek said the Communist Party, whose Prague city committee resigned Saturday, had “ignored the people, the truth and everyday life” but that it now “wants to start on a new path.”

While he did not specifically mention the Civic Forum in his offer of talks, some analysts interpreted his remarks as signaling the possibility of an early meeting with opposition leader Vaclav Havel.

Speaking on national television Saturday night for the first time in 20 years, Havel said, “We’ll negotiate with the government if it’s ready to negotiate with us, but only at the highest level.” Earlier, Havel said the party had offered low-level talks.

Adding to the sense here that a new political alignment may be imminent were signs that the Civic Forum and caretaker Prime Minister Ladislav Adamec may be joining forces. Adamec resigned from the ruling Communist Party Presidium over the weekend, allegedly because “I don’t want to work with such people.” He also submitted his resignation as prime minister but stayed on in a caretaker role.

A spokesman for the Civic Forum said at a press conference Saturday night that nine of its members would meet with Adamec today. Earlier Saturday, Havel urged support for Adamec at a rally that attracted a crowd officially estimated at 500,000 to 800,000.

Television, which had stuck closely to the party line through most of Friday, broke a 20-year precedent Saturday with extensive live coverage of both the giant Prague rally and a second demonstration that drew about 60,000 people in the Slovak capital, Bratislava.

Events here have now developed such a powerful momentum that workers at both the state television network and the Ministry of Education announced they will join a planned nationwide general strike Monday that is billed as a referendum on Communist rule.

“We are no longer the opposition,” Civic Forum representative Michal Horacek said. “They are the opposition.”

A clearly shaken Urbanek spent his first day on the job witnessing the resignation of the entire Prague city Communist Party committee, including its unpopular boss, Miroslav Stepan.

“They leaned over a long way by the standards of this arthritic political system,” a Western diplomat commented on the party’s weekend leadership purge.

But the changes were far from enough to satisfy the sea of cheering Czechoslovaks who filled Letna parade ground, Prague’s largest open space, on Saturday, waving national flags and protest banners.

“The new leadership is only a trick meant to confuse us,” Havel told the masses in a voice hoarse from days of strain. “The power remains . . . in the hands of the neo-Stalinists.”

Havel urged that his compatriots increase their pressure on the government with Monday’s two-hour nationwide protest strike--and by preparing for a possible open-ended strike later. “The strike is the most important thing we should focus on at the moment,” the popular writer urged as protesters jumped up and down on a subfreezing afternoon punctuated by snow flurries.

“All that has been opened slightly should now be opened wide,” Havel added.

The demonstrators kept remarkable order during a nearly three-hour rally, moving when asked almost as a single body in order to relieve the crush at the front of the crowd--and all without any noticeable police presence.

The city was covered with an explosion of homemade posters, announcements and photographs of the last time the police made their presence felt--brutally suppressing a much smaller, peaceful gathering only nine days before.

Earlier Saturday, Cardinal Frantisek Tomasek, the 92-year-old archbishop of Prague, told worshipers, “In this important hour of struggle for truth and justice in this country, I and the Catholic Church are on the side of the nation.”

The elderly cleric, long known for his caution in dealing with the authorities, has become progressively more outspoken as members of the church have begun to press the authorities for more religious and other freedoms.

“I must not be silent at the moment when you have united in a mighty protest against the injustice committed against us for four decades,” he said in a statement released earlier this week and reiterated from the altar of Prague’s St. Vitus Cathedral on Saturday.

An estimated 40% of Czechoslovakia’s 15 million citizens are Roman Catholics.

Regional Communist Party committees were holding emergency meetings Saturday, and one in Prague was particularly critical, with members coming out in support of Monday’s strike and demanding that the weekend’s national party leadership meeting be immediately reconvened to get rid of still more hard-liners.

More than 1,000 delegates from neighborhood and factory party organizations from the capital whistled down the city’s party boss, Stepan, and engaged in an unprecedented, no-holds-barred clash of views. Stepan was considered unlikely to retain his Presidium post in the national Communist Party leadership after losing his power base in Prague.

“I’ve been a member of the Communist Party for 26 years and I’ve never seen anything like this,” commented a delegate who works in the government’s foreign trade section. “This clash of opinion was needed 20 years ago.”

Czechoslovak President Gustav Husak announced Saturday night an amnesty for all political prisoners that Adamec, as part of his bid to gain popular support, had proposed earlier in the day.

Husak, who served as Communist Party chief from soon after the 1968 Soviet-led invasion until 1987, was one of those who lost his seat on the party Presidium at the weekend party meeting, and he is considered likely to lose the mostly ceremonial presidential post as well.

Adamec also said in a television interview Saturday that the government would speed up work on new laws regarding freedom of assembly, association and the press, as well as a new constitution.

An excited Civic Forum activist, Martin Poulous, exclaimed, “I think we’re winning.”