CHRONOLOGY-Ukraine's politics since Orange Revolution

(Reuters) - Ukrainian President Viktor Yushchenko's decree to dissolve parliament came into force on Tuesday, a move immediately challenged in the courts by his arch rival, Prime Minister Viktor Yanukovich.

Here is a chronology of events since the "Orange Revolution" which helped sweep Yushchenko to power.

Nov 22, 2004 - Supporters of Yushchenko, opposition leader demonstrate in Kiev -- dressed in orange -- after results show Prime Minister Yanukovich in the lead in the second round of a poll to decide who will succeed President Leonid Kuchma.

Dec 3 - Supreme Court rules the vote invalid and orders second round re-run on December 26 after parliament votes to sack Yanukovich government.

Dec 28 - Yushchenko wins the election with 51.99 percent. Supreme Court rejects complaints by Yanukovich of poll violations and he resigns as prime minister on New Year's Eve.

Jan 23, 2005 - Yushchenko sworn in as president. Days later, Yulia Tymoshenko is approved as prime minister by parliament.

Sept 8 - Yushchenko sacks Tymoshenko government following resignations of two top officials. Yuri Yekhanurov, a loyal presidential ally, takes her place.

Jan 10, 2006 - Parliament votes to sack the government over a gas deal with Russia sharply raising the cost of imports.

March 26 - Yanukovitch's Regions Party, more sympathetic to Moscow, comes first in parliamentary elections with 186 of 450 seats, but is outnumbered by the combined "orange" score of 243. Tymoshenko's bloc has the best score of "orange" groups.