Ndayambaje and his five co-accused colleagues have denied genocide and crimes against humanity in the largest and longest trial before the UN Court.
The accused asserted that he ceased to occupy the position of Mayor in 1992, after serving in the post for nine years, on his own conviction as he wanted to pursue further studies.
"I voluntarily chose to step down in order to give myself time for studies," he stressed before three bench judges made up of William Sekule of Tanzania (Presiding), Arlette Ramaroson (Madagascar) and Solomy Bossa (Uganda).
Ndayambaje, 50, continued Monday afternoon his testimony.
Some 24 witnesses have testified for the defence of Ndayambaje when the trial was adjourned on 24 September.
The defendant was re-appointed as the Mayor on 22 June, 1994 at the peak of the 1994 killings.
The trial started in June 2001 and is expected to close before end of the year.
According to UN estimates about 800,000 ethnic Tutsis and moderate Hutus were killed in the April-July slaughter.
© Hirondelle News Agency