Lawyers for the jailed Ukrainian opposition leader Yulia Tymoshenko have begun putting her complaint against imprisonment to judges in Strasbourg.
Tymoshenko, a former prime minister, says Ukraine's prosecution of her was politically motivated and her detention unlawful. She is now being treated for acute back pain in a state hospital.
Last October she was convicted of abuse of office and jailed for seven years.
President Viktor Yanukovych has been her arch-rival for nearly a decade.
The European Court of Human Rights in Strasbourg held a half-day public hearing before embarking on an in-depth study of her case behind closed doors.
If Ukraine's judiciary is found to be at fault the court can impose penalties on Ukraine, a signatory to the European Convention on Human Rights.
With her distinctive plaited, blonde hair Tymoshenko was a key figure in Ukraine's 2004 Orange Revolution. Since then she has twice served as prime minister.
Many EU politicians have echoed her criticisms of the Ukrainian authorities and in June European leaders boycotted Euro 2012 football matches in Ukraine, to show their displeasure at her detention.
Tymoshenko argues that her detention was politically motivated and that there has been no judicial review. She also says the authorities neglected her medical needs and kept up round-the-clock surveillance after moving her to a hospital in the eastern city of Kharkiv.
Run-up to election
On Wednesday the Ukrainian high court is expected to rule on her appeal against conviction.
She was found guilty of abuse of office over a gas deal she signed with Russia's Vladimir Putin.
Lawyers for the state of Ukraine dispute her complaint, and will argue in Strasbourg that it was just a normal criminal trial, the BBC's Moscow correspondent Daniel Sandford says.
The Tymoshenko case is likely to loom large in Ukraine's parliamentary election on 28 October, when Mr Yanukovych's Party of the Regions will seek to maintain its grip on parliament.
In a separate trial, Tymoshenko is accused of embezzlement and tax evasion in connection with business deals she did in the 1990s.
In the Orange Revolution pro-Western opposition activists angry at official corruption and cronyism prevented Mr Yanukovych taking office after an election widely condemned as rigged.
Mr Yanukovych has sought to renew the close ties with Russia that existed in Soviet times - a stance that alarms many Ukrainian nationalists.