Kasab, a Pakistani national, was arrested in November after more than 170 people were killed and 300 injured in a three-day killing rampage through India's commercial capital by 10 armed men.
Abbas Kazmi, Kasab's new lawyer, told the judge: "I am willing to appear for the accused. It is an honour, as we must present a stable and democratic view of our nation."
He has asked for government protection, saying he is concerned about his safety.
The Mumbai Metropolitan Magistrate Court's Bar Association had previously resolved not to represent Kasab, while some lawyers who said they were willing to take on the case had their homes attacked.
The trial is taking place inside a high-security prison because of security fears.
Kasab is said to belong to the banned Pakistan-based group, Lashkar-e-Taiba.
Prosecutors say they have evidence that "undoubtedly and conclusively" links the attacks to Pakistan, including mobile and satellite phone communication between the armed attackers and their [Lashkar] "handlers".
Ujjwal Nikam, the special public prosecutor, told the court on Friday that at least one Pakistani military officer was involved in the attack and its sophistication suggested the involvement of Pakistan's powerful intelligence agency.
"There was a criminal conspiracy hatched in Pakistan to attack India," Nikam said, with the "ultimate target of capturing Jammu and Kashmir, which is part and parcel of India".
The Himalayan region of Kashmir, which is divided between India and Pakistan, but claimed by both, has long been at the centre of the bitterness between the two South Asian rivals.
Kasab's DNA and fingerprints were found on items retrieved from the hijacked Indian fishing trawler the men used to get to the Mumbai coast, it is alleged.
There is CCTV and other footage allegedly of Kasab at Mumbai's main railway station, where more than 50 people were killed when he and another attacker opened fire with AK-47 assault rifles and threw grenades.
Thirty witnesses also picked him out in identification parades, the charge sheet said.
Kasab could face the death penalty if convicted of the charges against him.
Two Indian nationals are also on trial on charges of providing the group with logistical support before the attacks.