The first of Pol Pot's comrades has been formally charged, more than 30 years since the start of Cambodia's genocide.

Chief Khmer Rouge prison guard Duch was charged with crimes against humanity today, accused over the deaths of 1.7m people in Cambodia's 'Killing Fields'.

After a morning hearing before a tribunal of domestic and international judges, the 65-year-old schoolteacher and born-again Christian was taken into provisional custody of the court.

The long-awaited €41m Cambodian-United Nations tribunal into atrocities committed during the Khmer Rouge's 1975-79 reign of terror has its own detention centre on the outskirts of Phnom Penh.

Duch, also known as Kang Kek Ieu, has confessed to committing multiple atrocities during this time as head of the capital's notorious Tuol Sleng, or S-21, interrogation centre.

He is expected to be a key witness in the trial of other senior Khmer Rouge cadres and had been in a military prison in the southeast Asian nation's capital since 1999.

At least 14,000 people deemed to be opponents of Pol Pot's 'Year Zero' revolution passed through Tuol Sleng's barbed-wire gates. Fewer than ten are thought to have lived to tell the tale.   

Most victims were tortured and forced to confess to a variety of crimes - mainly being CIA spies - before being bludgeoned to death in a field on the outskirts of the city. Women, children and even babies were among those butchered.

Earlier this month, prosecutors lodged formal cases against five suspects, who have not been named.

Besides Duch, they are widely thought to be 'Brother Number Two' Nuon Chea, Khmer Rouge foreign minister Ieng Sary, former president Khieu Samphan and Meas Muth, a son-in-law of Pol Pot's military chief Ta Mok, who died last year.

'Brother Number One' Pol Pot died in April 1998 in Anlong Veng, a final Khmer Rouge redoubt in jungle-clad mountains along the Thai border.