One of Myanmar's most prominent political prisoners will be freed on Friday in an amnesty for 651 prisoners, a prison official said, and activists expect many more released throughout the day.

Min Ko Naing, who played a leading role in a pro-democracy uprising in 1988 in which thousands of protesters were killed, will be freed from Thayet Prison, a prison official and his a member of his family told Reuters.

He was arrested in 1989, freed in 2004, and then arrested again in 2007 on charges of organising protests.

Activists said they expected more political prisoners to be among those freed in Friday's amnesty, a step that could help pave the way for the lifting of economic sanction by the European Union and soon thereafter by the United States.

An official from Thayet Prison told Reuters Min Ko Kaing, 49, will be among the 651 prisoners released in Friday's amnesty. Kyi Kyi Nyunt, elder sister of Min Ko Naing, confirmed her brother would be freed.

They have just told me that my brother will be freed at about 8 a.m., she said, adding she had sent family members to meet her brother at prison.

Secluded Myanmar has initiated radical reforms since a civilian government was allowed to take power in March after almost half a century of rule by the military. Some political prisoners have been freed, media curbs eased and the government has initiated a dialogue with Nobel laureate Aung San Suu Kyi.

U.S. Secretary of State Hillary Clinton visited Myanmar in early December and promised to support more aid and consider installing an ambassador.

The next major step in the reform process will April by-elections already announced by the government, in which Suu Kyi and her National League for Democracy (NLD) will take part.

The United States and Europe have said freeing political prisoners is crucial to even considering lifting sanctions that have isolated the former British colony also known as Burma and, over the years, pushed it closer to China.

Myanmar freed about 230 political detainees in a general amnesty on October 12. A senior political adviser to President Thein Sein told Reuters in November that hundreds of political detainees may soon be released.

The exact number behind bars is unclear.

Rights groups and the United Nations have put it at about 2,100. But Minister for Home Affairs Lieutenant General Ko Ko told U.N. Special Rapporteur Tomás Ojea Quintana in August the number was 600, or about 400 after the October 12 amnesty.

The Assistance Association for Political Prisoners (Burma), a group that tracks prisoners, had identified more than 1,000 political prisoners. But diplomats and some independent analysts question those numbers and say they depend on different definitions of political prisoners and whether armed rebels or those who used force to oppose the government are included.

A review of the AAPP's list of prisoners by European diplomatic missions in Thailand suggested the number of non-combatant prisoners of conscience appeared to be about 600, according to diplomats, or about 800 before October's amnesty.

Rights group Amnesty International dropped its earlier estimates of about 2,000 political prisoners due to conflicting definitions of prisoners.

(Reporting by Aung Hla Tun. Writing by Jason Szep; Editing by Jonathan Thatcher)