He said that the international community often ignores the Palestinian "state-sanctioned culture of incitement, the glorification of terror, the daily broadcasts that call on Palestinians to kill, stab, shoot and run over Israelis".
Mr Cotler spoke while attending the Sergei Magnitsky Human Rights Awards in London, where he was honoured for his life's work. The lawyer's famous clients have also included Natan Sharansky, the Soviet political prisoner who went on to become the Jewish Agency's chairman.
The awards event was set up by businessman Bill Browder to mark the death of his lawyer, Sergei Magnitsky, who was unlawfully killed in Russia six years ago after uncovering corruption in the country.
Mr Cotler revealed that his battle to secure justice for Mandela in 1981 landed him in trouble with the South African government of the time.
He explained how, after delivering an impassioned anti-apartheid speech, he was arrested by officials who were waiting for him in the audience.
He was then taken to the office of then foreign minister Pik Botha, who wanted to talk to the man who defended his hero, Natan Sharansky.
Mr Botha pointed to a picture of Mr Sharansky, hanging on his wall.
Mr Cotler said: "He looked at me and said: 'I cannot understand how somebody like you, who can defend this great hero who is fighting against our common enemy, the Communist Soviet Union, can, in the same breath defend Nelson Mandela, who is also our enemy.'
"I said to him: 'Sharansky and Mandela are fighting for freedom and democracy.'"
After several hours of debate, the two men came to an agreement. Mr Botha agreed to release the Canadian Jewish lawyer for 10 days - during which Mr Cotler was instructed to travel across the country in order to verify the foreign minister's claim that South Africa was a true "pluralist democracy".
"I came back and told Mr Botha: 'South Africa is democratic - if you are white. Now, I am going to fight harder. I am going to fight this racist regime until it is defeated.'" And he did.
Mr Cotler is still in touch with Mr Botha, who went on to call for Mandela's release and even joined the African National Congress.
But he is also still in contact with the anti-apartheid campaigners, who now back an Israel boycott.
Does he? "As somebody who understands the pervasive evil of apartheid, to say 'Israel is an apartheid state' is not only false and prejudicial to Israel, but it undermines the real struggle against apartheid and the integrity of that movement."
Mr Cotler, an emeritus professor at the McGill University in Montreal, says his drive for justice and human rights stems from his Jewish values.
He said: "I have always been anchored in the teachings of my parents. My father would say: 'Tzedek, tzedek tirdof' ('Justice, justice you shall pursue')."
Mr Cotler retired as a member of Canada's parliament last month.
