Four former student activists were acquitted in a retrial on Monday 39 years after they were imprisoned for campaigning against the military regime led by then President Park Chung-hee.

In 1974, the four were sentenced to prison terms ranging from seven to 10 years for leading a protest against the revision of the Yushin Constitution that the former president introduced to prolong his stay in power, in violation of a now-defunct emergency decree.

In the retrial, the Seoul High Court delivered a not-guilty verdict to the former students of Sogang University, including Yim Sang-woo who is now a history professor at the school, saying emergency decree No. 1 is defunct and unconstitutional.

The decision was made in line with a landmark ruling by the Constitutional Court in March that three emergency decrees issued by the former president were unconstitutional as they excessively violated people's basic rights.

Park, who ruled the nation from 1961-1979 after seizing power in a military coup, is the father of President Park Geun-hye.

The elder Park proclaimed presidential emergency decree No. 1 in 1974 banning all activities aimed at revising the Yushin Constitution and putting any offenders in prison for up to 15 years. (Yonhap News)