Ukrainian President Viktor Yushchenko, who is locked in a power struggle with his prime minister, has appealed to the Constitutional Court against a law weakening his powers, his office said Tuesday.
The law was supposed to help clarify the division of power between the president's office and Prime Minister Viktor Yanukovych's Cabinet.
But the pro-Western Yushchenko, who has seen his influence ebb away in recent months, has complained that it gives more authority to the Russian-leaning Yanukovych.
"It is a violation of the Constitution. It is a usurpation of power," said Yushchenko.
Under the law which came into force last week, Yushchenko sees his right to appoint the foreign and defense ministers limited, and his influence over regional leaders weakened.
The law also gives legislators the right to appoint the premier without the president's approval if the president does not approve parliament's nominee within 15 days.
Last month, Yushchenko proposed 42 changes to the disputed law, but parliament rejected all of his suggestions and lawmakers overrode his veto of the bill.
Yushchenko has shortened his list of proposed changes to eight and the parliamentary coalition said it would consider them on Thursday.
The law is the first to come into force without the president's signature.
After the Kremlin-backed Yanukovych was accused of vote-rigging in the 2004 presidential election, "Orange Revolution" protests helped pave the way to Yushchenko's election victory.
But the two men had to share power after Yanukovych won last year's parliamentary election and was named prime minister.
Last week, pro-Western Foreign Minister Borys Tarasyuk resigned after a monthlong struggle with the Yanukovych government that at one point saw the Cabinet cut off funding for the Foreign Ministry, stopping it from paying diplomats' salaries, reports AP.
Yushchenko on Monday asked parliament to approve career diplomat Volodymyr Ohryzko as the new foreign minister, but he is seen as close to Tarasyuk and an ally of the prime minister.
Since he became prime minister, Yanukovych has suspended the president's goal of NATO membership, expressed interest in joining a Russian-led ex-Soviet trade bloc and responded favorably to Moscow's proposals for joint ownership of Ukraine's gas transit pipeline network.