"Brother Number Two" Nuon Chea urged Cambodia's UN-backed court to ignore testimony by Kaing Guek Eav, better known as Duch, who accused him of ordering the "smashing" of hundreds of inmates at the S-21 detention centre in the late 1970s.
"I would like to inform the Cambodian people that I have never at any time been responsible for the operation of S-21," said the former deputy leader of the brutal regime, which oversaw the deaths of up to two million people.
"What Duch has accused me of has been untruthful and very unjust towards me... I have never been Duch's superior," Nuon Chea read out from a prepared statement, challenging his accuser to provide the documents to prove otherwise.
The 85-year-old, as usual dressed in all black, then refused to answer any questions, to the annoyance of the prosecutors, who urged judges to give "no weight to his exculpatory assertions".
Nuon Chea and his co-defendants -- ex-foreign minister Ieng Sary and one-time head of state Khieu Samphan -- deny charges of war crimes, crimes against humanity and genocide.
Former maths teacher Duch, 69, was sentenced to life in jail on appeal in February for overseeing the killings of some 15,000 people at the notorious prison.
Nuon Chea and Duch, who are being held at the same detention centre, are on bad terms, with the former calling Duch "rotten wood" during a recent hearing.
Duch told the court earlier this month that Nuon Chea has long blamed him for failing to destroy evidence at S-21 before the Vietnamese invaded and ousted the Khmer Rouge.
Led by "Brother Number One" Pol Pot, who died in 1998, the Khmer Rouge wiped out nearly a quarter of Cambodia's population between 1975 and 1979 through torture, starvation, overwork or execution in a bid to forge a communist "utopia".