President Volodymyr Zelensky has signed a bill that critics say weakens the independence of Ukraine's anti-corruption bodies, sparking protests in several cities and drawing international criticism.

The new law grants the prosecutor general control of the National Anti-Corruption Bureau (Nabu) and Specialised Anti-Corruption Prosecutor's Office (Sap), and critics say it undermines their authority.

In an address on Wednesday, Zelensky said both agencies would still "work" but needed to be cleared of "Russian influence".

After the bill passed, hundreds of people gathered in Kyiv for the biggest anti-government protest since the start of Russia's full-scale invasion in February 2022.

Demonstrations were also seen in the cities of Lviv, Dnipro and Odesa.

"We chose Europe, not autocracy," said a poster held by one demonstrator. "My father did not die for this," said another.

Ukraine's chief prosecutor, Zelensky loyalist Ruslan Kravchenko, will now be able to reassign corruption probes to potentially more pliant investigators, and even to close them.

In his overnight address, Zelensky criticised the efficiency of Ukraine's anti-corruption infrastructure, saying cases had been "lying dormant".

"There is no rational explanation for why criminal proceedings worth billions have been 'hanging' for years," he said.

He added that the prosecutor general would ensure "the inevitability of punishment" for those who broke the law.

To critics of the legislation, this is at odds with more than a decade of pro-democracy and anti-corruption efforts.

It is such aspirations that led to the Euromaidan uprising and the downfall of pro-Russian president Viktor Yanukovych in 2014, followed by Russia's invasion and the deaths of hundreds of thousands people.

The Ukrainian government has cited Russian influence for curtailing Nabu's powers.

The day before the controversial law was passed, Ukraine's Security Service and the prosecutor general's office carried out searches and arrests targeting alleged Russian spies at Nabu.

The move has sparked concern among Ukraine's Western allies.

The Ukrainian independent anti-corruption system was set up at their insistence and under their supervision 10 years ago.

It was a key precondition for their aid and stronger ties as Ukraine declared a pro-democracy course amid Russia's initial invasion of 2014.

"The European Union is concerned about Ukraine's recent actions with regard to its anti-corruption institutions," says European Commission spokesperson Guillaume Mercier.

"The EU provides significant financial assistance to Ukraine, conditional on progress in transparency, judicial reform, and democratic governance."

Stamping out corruption is also a key requirement for Ukraine's application to join the EU.

The European commissioner for enlargement, Marta Kos, criticised the bill's adoption.

"The dismantling of key safeguards protecting Nabu's independence is a serious step back," Kos wrote on social media, saying the two bodies were "essential" for Ukraine's EU path.

Ukraine's deputy prime minister for European and Euro-Atlantic integration, Taras Kachka, said he had assured Marta Kos there would be no compromise on anti-corruption, and that "all core functions remain intact".

Dmytro Kuleba, who stood down as foreign minister last year, said it was a "bad day for Ukraine" and that the president had a choice - either to stand on the side of the people or not.

Ambassadors from the G7 group of nations have said they would like to discuss the pressure on Nabu and Sap with the Ukrainian leadership.

However, Ukraine's allies would be extremely reluctant to roll back the aid and co-operation at a time when its troops are struggling on the frontline.