A former top political aide to the Ukrainian President, Viktor Yushchenko, has accused several senior government officials of systematic corruption.
Mr Zinchenko warned of a possible "counter-revolution"
Oleksandr Zinchenko said those officials were undermining the goals of the Orange Revolution that swept Mr Yushchenko to power in January.
"Corruption is now even worse than before," said Mr Zinchenko, who quit as state secretary on Saturday.
He is the first top official to resign since Mr Yushchenko's poll triumph.
The surprise move is seen by some analysts as a sign of a struggle within Mr Yushchenko's team, over influence and access to the president.
"With my resignation I am trying... to make both the president and his team understand the grave danger of the current situation," Mr Zinchenko told a news conference in the capital, Kiev.
"Now corruption and bribe-taking are growing in force," he said.
Mr Zinchenko specifically named Ukraine's National Defence and Security Council head Petro Poroshenko and senior presidential advisor Olexandr Tretyakov.
He accused the two men of nepotism and also pressure on the judiciary.
He warned that the president could face a "counter-revolution" if he did not fire Mr Poroshenko and Mr Tretyakov.
Mr Poroshenko - who was present at the news conference - later denied the claims, challenging Mr Zinchenko to back up his comments with facts.
There have been reports of a growing infighting in Mr Yushchenko's team ahead of parliamentary elections in March.
Mr Yushchenko came to power pledging to make the fight against corruption one of his top priorities.
He has recently come in for strong criticism as a result of a reporter's investigation into his son's alleged life of luxury.