Members of Ukraine’s Parliament saw things differently a decade ago. In 2004, they voted to expel Mr. Tyagnibok over a speech in which he described World War II-era partisans bravely fighting Germans, Russians, Jews and “other scum.” He went on to slur what he called the “Jewish-Russian mafia” running Ukraine.

Until 2004, Svoboda had been called the Social-Nationalist Party, which critics said was just a word flip away from its true ambitions and a deliberate reference to the National Socialism of the Nazis. Unabashed neo-Nazis still populate its ranks, organizations that study hate groups in Europe say.

Svoboda never won more than a fraction of a percent of the national vote, in spite of having strongholds in city councils and regional legislatures in its base in western Ukraine. Its fortunes changed with the election of Mr. Yanukovich. Serhiy Rudyk, a party official, said the new president’s pro-Russia policies angered Ukrainians, helping Svoboda in the ballot box.

Critics of the party’s role in Ukrainian politics have another explanation. The party, they say, drew strength from an orchestrated policy of Mr. Yanukovich to foster a right-wing competitor to his main political rival, Yulia V. Tymoshenko, who had previously enjoyed strong support in the country’s west.

In 2011, for example, Mr. Yanukovich’s supporters unfurled the flag of the Soviet Union during marches in Lviv on Victory Day, a holiday that commemorates the end of World War II, despite a municipal law banning the display of Communist flags in the city limits. It was a wedge issue that gave Svoboda a lift in the polls. Svoboda denies this assessment, and it is a stated ally of Ms. Tymoshenko.

The next year, however, the party won 8.5 percent of the seats in Parliament, provoking warnings from Israel about rising anti-Semitism and xenophobia in Ukraine, a country with a rich history of both. On their first day in Parliament, Svoboda lawmakers started a fistfight with members of Mr. Yanukovich’s party.

The party, critics say, became something of a Frankenstein’s monster for Mr. Yanukovich, and it has grown beyond all expectations with its activists now playing an integral role in the barricading of Independence Square.