KYIV -- Kateryna Handzyuk, a Ukrainian civic activist and adviser to the mayor of the Black Sea port city of Kherson, has died from wounds she suffered from an acid attack three months ago.

The 33-year-old Handzyuk died on November 4 in a Kyiv hospital where she was being treated for burns from the attack, colleagues and officials said.

Local media suggested that Handzyuk's death was caused by a blood clot.

Several hundred supporters gathered around Ukraine’s Interior Ministry building in Kyiv late on November 4, demanding that those responsible for her death be brought to justice.

The activist, who was known for her scathing criticism of police corruption, was doused with sulfuric acid outside of her Kherson home on July 31 by an unknown attacker.

Her death comes amid a wave of attacks against Ukraine’s civic activists, with rights campaigners claiming law-enforcement agencies have failed to thoroughly investigate the cases and may even be complicit in some of the attacks.

Ukrainian President Petro Poroshenko, speaking during his trip in Turkey, expressed his condolences to Handzyuk’s family and called on law-enforcement agencies to do everything in their power to bring her killer to justice.

Five suspects have been detained for their alleged involvement in the attack, but there was no information about its mastermind.

"Attacks against civil society activists are unacceptable. The perpetrators of this vicious crime must be brought to justice," EU enlargement commissioner Johannes Hahn tweeted.

Handzyuk suffered severe burns to nearly 40 percent of her body and lost sight in one of her eyes after the acid attack, according to doctors who treated her at a burn center in the Ukrainian capital, Kyiv.

Doctors performed 11 surgical operations to try to save her life. From her hospital bed, Handzyuk vowed to track down her attackers.

Police initially listed the case as hooliganism but changed it to attempted murder committed with extreme cruelty after public outcry.

WATCH: Handzyuk speaking from her hospital bed

Ukrainian lawmaker Olena Sotnyk on November 4 renewed her previous call for a special investigative committee to be formed in parliament to probe her case.

Local and international civil society groups have recorded at least 55 unsolved attacks against activists, including on Handzyuk, since 2017.

In recent months, protesters demanding a proper police response have gathered outside government buildings across the country in a campaign dubbed “silence kills.”

Yuriy Lutsenko, Ukraine’s prosecutor general and a presidential appointee, caused uproar after one of the protests in September, when he said activists were themselves partly to blame because they “stir up” an “atmosphere of total hatred toward the authorities.”

Handzyuk was stinging in her criticism of police corruption.

In September 2017, she accused Artem Antoshchuk, a department head in the Kherson Regional Police, of demanding a 3 percent cut from all contracts and tenders in the region.

The accusation led to a fierce court battle, which she won.

Police have arrested five former fighters of the Ukrainian Volunteer Army, a splinter faction of the ultranationalist Right Sector militia, suspected of involvement in the attack.

Four of the men have claimed the fifth, Serhiy Torbin, a former officer of Kherson police, was the main suspect.

Torbin is in the custody of the Security Service of Ukraine at a pretrial detention center in Kyiv, his defense lawyer Yuriy Khazov told the Kyiv Post newspaper.

Stills from a CCTV camera published by local media appear to show the alleged attacker running away from the scene of the crime.

Six weeks before her death, Handzyuk recorded a video message for Hromadske TV from her hospital bed. Wrapped in bandages, she said she was certain the attack was meant to kill her.

“Why do I consider it to be assassination attempt? Because the acid was poured on my head,” she said. “If someone wanted to warn or silence me, they could have targeted my arms, legs, or face -- anywhere. But they poured a liter of acid on my head.”

Before she signed off, she added: “Yes, I know that I look bad now. But I’m sure that I look much better than law and justice in Ukraine,” she said. “Because they aren’t treated by anyone.”