Opposition leader Chokri Belaid, secretary-general of the Democratic Patriots Movement, was assassinated Wednesday on his way to work in Tunis. Pillay said from Geneva she was saddened to hear the news, which followed similar events last year.
"I call on all actors in government and civil society to unite strongly behind Belaid's campaign against political violence," she said in a statement. "That would, at least, provide him with a fitting memorial for his invaluable work as a human rights defender and opponent of violence."
A protest suicide in Tunisia in December 2010 sparked the Arab Spring and ushered in a new government in the country. Pillay said Belaid's assassination comes at a time when political violence in Tunisia is at a historic high, however.
Eric Goldstein, deputy director of Middle East programs at Human Rights Watch, said although Tunisians drafted a new constitution, political gains are slow to develop.
"Tunisians have learned they must remain vigilant to secure the rights for which they fought a revolution two years ago," he said in a statement.