WARSAW, SEPT. 17 -- Solidarity union leader Lech Walesa, who orchestrated the peaceful revolution that crushed communism in Poland and now is challenging the democratic government he helped create, formally announced today that he is running for president.

"Today, I made up my mind. I am putting forward for society's approval my readiness to be a candidate for the post of president of the Polish Republic," said Walesa, 46, the former shipyard electrician and Nobel Peace Prize winner who a decade ago became chairman of the first independent labor union in the Communist East Bloc.

Walesa described his candidacy, which comes after months of broad hints and half-hearted denials, as "the fulfillment of the oath" he took in 1980 as Solidarity chairman. Then, he vowed to overthrow communism and establish full democracy.

When Solidarity assumed control of the government last year, Walesa opted to take no formal office himself. Instead, he returned home to the Baltic port of Gdansk -- from where he soon began finding fault with a government run by his former Solidarity colleagues.

Walesa accuses that government -- led by Prime Minister Tadeusz Mazowiecki, who was handpicked for that job by Walesa -- of moving too slowly to purge former Communists from positions in government and state industry.

He also is demanding the resignation of President Wojciech Jaruzelski, the longtime Communist leader and army general who imposed martial law here in 1981. Jaruzelski was chosen president last year as part of a power-sharing arrangement between Solidarity and the Communists.

Walesa has complained that Poland, which was the first country in Eastern Europe to break the Communist monopoly on political power, is now the only one not to have had totally free presidential and parliamentary elections.

Prime Minister Mazowiecki, a former journalist and longtime adviser to the Solidarity chairman, has emerged as a somewhat reluctant rival.

The prime minister, a cautious man who has won wide public support for his handling of the government's free-market economic program, consistently outscores Walesa in public opinion polls. Those surveyed say Mazowiecki is a more trustworthy and responsible leader than Walesa.

Mazowiecki is being urged to run by technocrats and professionals in major cities. They claim that Walesa is a demagogue with a dangerously dim grasp of the difficulties of transforming a debt-ridden socialist state into a capitalist European nation.

Walesa's opponents accuse him of posturing and of using loaded language designed to appeal to antisemitic and ultra-nationalist sentiments. Walesa, dismissing his critics as "eggheads," angrily denies that he or his supporters are antisemitic or nationalistic.

A source in Mazowiecki's office said today that the prime minister will probably announce this week that he will run against Walesa.

By making his own announcement today, Walesa appears to be trying to take charge of what is expected to be a pivotal week in the history of post-communist Poland.

Major political power brokers will meet on Tuesday under the auspices of the Roman Catholic Church to discuss a timetable for new presidential and parliamentary elections, as well as for the resignation of Jaruzelski. Cardinal Jozef Glemp, the Polish primate, is to preside.

Walesa, Mazowiecki and Jaruzelski have said they will attend. Key members of parliament also are expected, along with leaders of the two nascent political parties that have formed around the candidacies of Walesa and Mazowiecki.

The round-table meeting will be followed by a session of parliament that is expected to schedule the presidential election for late this year. Parliamentary elections are not expected until next year.

Legislators say there is a strong likelihood that parliament will call on Jaruzelski to cut short his six-year term and resign immediately. Although he occupies a position that grants him significant authority, including the power to dissolve the government, Jaruzelski in the last year has played a quiet and supportive role within the Solidarity-controlled government.

Jaruzelski's office hinted indirectly last week that he may resign soon. The Polish president is refusing to give the White House specific dates for a long-planned state visit to the United States that had been announced for October. The visit has been condemned by Walesa's supporters as "an insult" to Poland.