Viktor Yanukovych has been sworn in as President of Ukraine following his victory in an election three weeks ago.
In a ceremony in parliament on Thursday, Mr Yanukovych, 59, vowed to defend the sovereignty and independence of the country.
Prime Minister Yulia Tymoshenko, says Mr Yanukovych won through fraud and refuses to recognise his victory.
The BBC reports neither Mrs Tymoshenko and outgoing President Viktor Yushchenko attended the event in Kiev.
International observers said the February poll was conducted fairly. Mr Yanukovych beat Mrs Tymoshenko in the run-off by 3.5%.
Mr Yanukovych, 59, acknowledged the divisions in parliament and Ukraine's economic difficulties, including "colossal debts, poverty and economic collapse".
But he said that he knew how to lead the country out of the crisis, urging the government and parliament to co-operate with him.
The BBC reports Ukrainian politics has been dominated by rivalries between Mr Yanukovych, Mrs Tymoshenko and Mr Yushchenko since the Orange Revolution five years ago.
Mass street protests at that time swept Mr Yushchenko and Mrs Tymoshenko to power.