Koufodinas, 59, was given the leave even though he is serving 11 life sentences plus 25 years for participating in 11 of the 23 assassinations perpetrated by the now-defunct November 17 group.
He arrived at Korydallos prison 90 minutes ahead of the noon deadline Saturday, accompanied by his wife and son, looking relaxed and waving to the press.
The United States, Britain and Turkey all criticized Greece for giving such a dangerous prisoner a two-day leave. Greece has defended the leave, saying it's allowed under the law. Opposition parties said the government should have blocked the move, noting that Koufodinas remains unrepentant about the slayings.
The November 17 group, which first appeared when its members assassinated the CIA station chief in Athens, Richard Welch, in December 1975, remained elusive until 2002, until one of its members was arrested after a botched bomb attack that year.
November 17 assassinated diplomats and military personnel from all three countries who protested the leave. They also killed Greek targets, including Pavlos Bakoyannis, a journalist and politician, husband of former Athens mayor and foreign minister Dora Bakoyannis and brother-in-law of current opposition leader Kyriakos Mitsotakis.
Both Dora Bakoyannis and Mitsotakis criticized the government for allowing Koufodinas, who gunned down Bakoyiannis in September 1989, to leave prison for the first time since 2002.
"I was in shock at first ... then, my first thought is not to have my children watch the news. Because one of the biggest trials is to have to explain what happened to the children," said Costas Bakoyannis, the governor of the central Greek region and Pavlos' son, who was 11 when his father was slain.
Koufodinas, who was convicted along with several other November 17 members in 2003, can apply again for a furlough after 60 days. While outside, he had to report twice daily to the nearest police precinct and was closely monitored.
Another convicted November 17 member, Christodoulos Xiros, did not return after his seventh furlough in December 2014 and had been planning terrorist attacks before he was re-arrested a year later.
Koufodinas said through his lawyer he used the furlough to reconnect with his family and plan to restart his beekeeping business, an activity he used as a front while operating as November 17's lead gunman, planner, recruiter and cashier.
Koufodinas, who has never disavowed his actions, described them in detail in a 2014 autobiography.
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