Belarus on Saturday unexpectedly freed leading rights activist Ales Belyatsky whose jailing for tax evasion in November 2011 prompted an outcry in the European Union and human rights circles.

Belyatsky, 51, who headed the rights group Vesna-96 and was serving a four-and-a-half year sentence, told journalists his release was a surprise to him.

"They let me go only an hour ago," he said after arriving in Minsk on a suburban train.

Belyatsky was arrested and charged after officials in EU members Poland and Lithuania unwittingly helped his prosecution by supplying information about bank accounts held in his name after a request by Belarus's financial authorities.

Belarus's authoritarian government imposes tough restrictions on the financing of non-governmental organizations and their activities that virtually rule out any financial help from abroad.

Under his leadership, Vesna-96 campaigned for scores of opposition activists prosecuted by the government of President Alexander Lukashenko.

At the time, the EU denounced Belyatsky's sentence as "clearly politically motivated" and said it symbolised an intensifying crackdown on Lukashenko's political opponents.

Lukashenko, who has ruled the ex-Soviet republic for 20 years, and other officials are subject to travel restrictions by the EU and the United States because of his harsh policies at home which have neutralized the political opposition.