Ukraine has been rocked by its largest protests since Russia's 2022 invasion after Volodymyr Zelensky loosened the country's anti-corruption rules.
Crowds of hundreds protested in central Kyiv late on Tuesday, with smaller demonstrations unfolding in several other cities.
In the West, diplomats have accused the country of "testing the limits more and more".
Volodymyr Zelensky's lawmakers earlier voted to impose tougher restrictions on anti-corruption agencies, and have brought them further under Government control.
The President then signed off on the move late on Tuesday.
Fighting corruption is seen as critical to erasing a legacy of Russian rule in Ukraine - and Zelensky's sign-off sparked fury at the protests and online.
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|Volodymyr Zelensky has imposed tougher restrictions on anti-corruption agencies
The National Anti-Corruption Bureau of Ukraine (Nabu) and the Specialised Anti-Corruption Prosecutor's Office will now be run by the general prosecutor - whom Zelensky appoints.
The head of Nabu had warned Zelensky not to approve the move, which he labelled an attempt to "destroy" Ukraine's anti-corruption rules.
One diplomat told Reuters the developments were "the most dangerous moment" yet for the independence of anti-corruption bodies.
"The Ukrainian side is testing the limits more and more," the bigwig said, referring to the patience of Kyiv's allies.
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|The head of Nabu had warned Zelensky not to approve the move, which he labelled an attempt to 'destroy' Ukraine's anti-corruption rules
EU enlargement commissioner Marta Kos, meanwhile, said she was "seriously concerned" by Tuesday's vote.
"The dismantling of key safeguards protecting Nabu's independence is a serious step back," she warned.
Throughout Russia's war, the agencies have brought corruption charges against MPs, ministers, and a former deputy head of Zelensky's administration.
But this month, an anti-corruption activist was charged with fraud and evading military service.
Critics said the charges were "political retribution" for exposing corrupt officials.
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|18-year-old protester Vladyslava Kirstyuk warned that the move brought back memories of Vladimir Putin's 2014 annexation of Crimea
Then this week, Ukraine's MI5 equivalent controversially arrested two Nabu officials over claims of "ties to Russia".
At the demonstrations, 18-year-old protester Vladyslava Kirstyuk warned that the move brought back memories of Vladimir Putin's 2014 annexation of Crimea.
"I know what it means for one person to have all the power, when nothing is transparent and everything is working against you," she said.
"I don't want it to be the same for us here."