Sophie Masala, a 55-year-old mother of two, appeared in the dock in Toulouse in a black suit and light-coloured blouse, occasionally shaking her head in disagreement as the indictment was read.
She admits to having killed Maryline Planche, 52, but insists it was not intentional.
Planche was killed at her apartment in central Toulouse in the south of France in May 2016, her head smashed with a bottle.
A few days later, a metal hacksaw was used to cut up her body.
From the apartment to the Canal du Midi, some 500 metres away, the body parts were transported in a supermarket trolley, and her head in a backpack.
The parts were found later, washed up along the banks of the canal.
A few days after the murder, Masala was arrested in her hometown of Montpellier about 250 kilometres away.
“This is an unusual trial… We hope that we will be able to understand by the end of it,” said a lawyer for Planche's family, Georges Catala.
The two women had worked at an association for handicapped people, where their dislike for each other was well known.
Judgment is expected on Friday, and Masala risks life imprisonment if convicted.