Ukraine's President Volodymyr Zelensky said that a suspect had been arrested in the shooting of former speaker of parliament Andriy Parubiy.
Mr Parubiy, a leading figure in the country's pro-European protest movements of 2004 and 2014, was shot dead on Saturday in the western city of Lviv.
Mr Zelensky said Internal Affairs Minister Igor Klymenko and security service chief Vasyl Maliuk had informed him of the arrest.
"The necessary investigative actions are ongoing," he posted on social media. "I thank our law enforcement officers for their prompt and coordinated work."
In a subsequent post, after having spoken to chief prosecutor Ruslan Kravchenko, he added: "The suspect has given an initial testimony."
"Urgent investigative actions are currently underway to establish all the circumstances of this murder," he said.
Mr Klymenko, posting on Telegram, said dozens of police officers and security officers had been involved in the operation to arrest the suspect, apprehended in the Khmelnytsky region of western Ukraine.
"There will not be many details now," he added.
"I will only say that the crime was carefully prepared: the schedule of the deceased's movements was studied, the route was laid, and an escape plan was thought out," he said.
On Saturday, hours after the shooting, Mr Zelensky had said that the shooting was a deliberate plot and had been carefully planned.
In the wake of the shooting, Ukraine's public broadcaster Suspilne cited anonymous sources saying the shooter had been dressed as a delivery rider and was on an electric bike.
Some tributes from Ukrainian officials to Mr Parubiy, who was still a member of parliament, hinted at suspicions of Russian involvement.
Since Russia's invasion of Ukraine began in 2022 both sides have accused each other of assassinations of key political and military figures.
Russian state media said Mr Parubiy has been wanted by Russian authorities since 2023.
Educated as a historian, Mr Parubiy had campaigned for Ukraine's independence from the Soviet Union as a young man.
He was also a major supporter of the use of the Ukrainian language over Russian - a highly politicised issue.
During the Maidan protests of 2014, he was a "commander" of opposition self-defence forces.
That same year, Ukrainian media said that he survived an assassination attempt by grenade.
After the ousting of then Ukrainian leader Viktor Yanukovych, who fled to Russia, Mr Parubiy served on the National Security and Defence Council for several months.
