LONDON -- Ukrainian President Volodymyr Zelenskyy is facing protests across the country after signing a controversial bill on Tuesday that critics say will neuter the independence of two prominent anti-corruption bodies.

As Ukrainian and Russian delegations prepare to meet in Istanbul, Turkey, for a new round of ceasefire talks, Zelenskyy and his allies are facing a groundswell of opposition at home.

On Tuesday, Zelenskyy signed a controversial law passed by parliament that will bring the Anti-Corruption Bureau (NABU) and its partner organization, the Specialised Anti-Corruption Prosecutor's Office (SAPO), under the direct control of the Prosecutor General's Office (PGO).

Both bodies were set up in the aftermath of Ukraine's pro-Western Maidan Revolution in 2014, with the intention of rooting out systemic corruption and helping Kyiv reform its democratic system with an eye on European Union accession.

The passing of the new legislation this week prompted protests in Kyiv and other major cities across Ukraine, with demonstrators even violating the nighttime curfew imposed as a guard against nightly Russian drone and missile strikes.

A spokesperson for the European Commission warned the move could undermine Ukraine's potential bid to join the EU. Kyiv's European funding, they added, is "conditional on progress on transparency, judicial reform and democratic government."

Transparency International's Ukraine branch, meanwhile, said the move represented a "massive setback in anti-corruption reform" and a "direct threat to Ukraine's path to the EU."

The passage of the bill followed dozens of raids on NABU employees by officers from the Security Service of Ukraine (SBU) and the PGO on Monday. Officers also began inspecting the handling of state secrets at SAPO.

Zelenskyy and his supporters have framed the measures as necessary to root out Russian infiltration and influence within Ukraine's anti-corruption bodies.

"The anti-corruption infrastructure will work," Zelenskyy wrote on Telegram late on Tuesday. "Only without Russian influences -- everything needs to be cleansed of this. And there should be more justice."

"Of course, NABU and SAPO will work. And it is important that the Prosecutor General is determined to ensure that in Ukraine the inevitability of punishment for those who go against the law is really ensured," he added. "And this is what is really needed for Ukraine. The cases that were pending must be investigated."

"For years, officials who fled Ukraine have been living peacefully abroad for some reason -- in very nice countries and without legal consequences," Zelenskyy continued.

"This is abnormal. There is no rational explanation why criminal proceedings worth billions have been 'hanging' for years. And there is no explanation why the Russians can still get the information they need."

"It is important that there is an inevitability of punishment and that society really sees this," the president wrote.

The bill was passed by the Ukrainian parliament -- the Rada -- by 263 representatives, having quickly moved through committee. Thirteen MPs voted against, 13 abstained and 35 did not vote.

It was supported in parliament by members of Zelenskyy's Servant of the People party, as well as former Prime Minister Yulia Tymoshenko's Batkivshchyna party. The Opposition Platform – For Life party, which is widely considered pro-Russian and has had several representatives accused of treason during the war, also backed the measure.

Oleksandr Merezhko, a member of Zelenskyy's party and the chair of the parliament's foreign affairs committee, told ABC News he backed the legislation though admitted he had "some doubts" about its content.

"I trust the president," Merezkho said. "In such cases I normally also trust the decision of the committee."

Merezhko said his concerns were over "what consequences it might have from the perspective of the negotiations with the EU on our membership."

European counterparts, Merezhko continued, "are worried and they are asking questions. I think that we need better communication with our European partners on that issue."

"There might be some sensitive aspects which need clear explanation to our partners by the president," he added. "I'm personally in favor of the independence of the anti-corruption bodies. But I'm also in favor of the true rule of law of the state in Ukraine."

Zelenskyy's decision to sign the divisive bill has piqued concern of an anti-democratic power grab by the president and his inner circle -- chief among them Andriy Yermak, the head of the presidential office.

A former Ukrainian official, who asked not to be identified for fear of reprisal, told ABC News, "We are not losing the war because the West did not give us enough weapons. We are losing the war because of corruption, lack of professional management and because many do not see why they should fight for Zelenskyy's autocracy."

"Yermak is just a good implementor of Zelenskyy's will," the former official added.

Vitaliy Shabunin, a prominent Ukrainian anti-corruption activist who previously headed the first Public Oversight Council at NABU, said the bill will allow the prosecutor general -- who is appointed by the president -- to "shut down all investigations involving the president's friends."

Kyiv Mayor Vitali Klitschko -- who has repeatedly clashed with Zelenskyy's administration during Russia's full-scale war -- joined protesters in the capital on Tuesday.

The new measure, he wrote on Telegram, "definitely does not bring Ukraine closer to the European Union. It certainly does not bring it closer to democracy, the rule of law, and legality -- to those values for which our soldiers are dying today in a bloody struggle against the aggressor."

Proponents of the bill "are dragging Ukraine faster and faster into authoritarianism," the mayor added, "hiding behind the war, destroying anti-corruption bodies, local self-government, silencing activists and journalists."

"Yes, there are many questions about the independence, impartiality, and adherence to legal procedures by all law enforcement agencies," Klitschko continued. "But the system needs to be changed, not turned into a bulldog of the authorities."

"And we must not forget that sooner or later, all actions will have to be accounted for -- both politically and legally," the mayor wrote.

ABC News' Will Gretsky contributed to this report.