The former premier is Croatia's highest official to be charged with corruption since 1991. Tackling graft was a key requirement when Croatia bid successfully to join the EU in 2013.
The Supreme Court in Zagreb on Thursday increased former prime minister Ivo Sanader's jail term from four-and-a-half to six years for taking more than €2 million ($2.2 million) in kickbacks from a real estate deal while he was in power from 2003 to 2009.
Sanader was also told by the court to return more than €2 million in gains from corruption.
He was taken away by police to start his term in Remetinac prison.
"This is politically motivated persecution that has lasted for nine years and I will still fight by all means to prove my innocence," Sanader told reporters before he was led away.
The 65-year-old was sentenced to two-and-a-half years in jail last October for war profiteering, and also faces charges in several other graft cases.
Zagreb building plot
The six-year sentence came from the prosecutor's appeal of Sanader's conviction in 2017. He was found by the court to have pushed his cabinet to buy a Zagreb building for the rural development ministry at twice its market value.
Of the €10 million paid by the government, €2.3 million was given directly to Sanader.
In order to qualify as a member of the European Union, Croatia has been taking steps to control corruption.
The Croatian State Prosecutor's Office for the Suppression of Organized Crime and Corruption (USKOK) says it has prosecuted 2,000 individuals.
Berlin-based anti-corruption NGO Transparency International ranked Croatia 57th on its 2017 Corruption Perception Index of 180 countries.
jm/rt (Reuters, AP)