Bulat Abilov, the co-chairman of Naghyz Aq Zhol (True Bright Path), and Peter Svoik, a leader of the unregistered opposition party Algha (Forward), made their announcement in Almaty.
The hunger strike comes after the Almaty Regional Court found all the leaders of the Naghyz Ak Zhol Party, the Algha Party, and the For a Just Kazakhstan Movement -- as well as some journalists -- guilty for organizing the gathering in Almaty on 26 February.
Punishments included jail terms ranging from 5 to 15 days, and fines.
The rally on 26 February saw thousands of people marching through the streets of Almaty to remember Ak Zhol Party co-Chairman Altynbek Sarsenbaev, who was found slain along with his driver and bodyguard on 13 February.
The head of the Kazakh Senate's administration and a group of secret-service operatives have been implicated in the slaying.
Also today, the United States condemned Sarsenbaev’s killing as "a serious crime" but urged calm.
State Department deputy spokesman Adam Ereli said the United States is assisting the government of Kazakhstan in an investigation.
"We are working with them to provide investigative assistance for the government, so it's an issue we're concerned about,” he said. “The government of Kazakhstan shares that concern. And we're working together to address it."
(with additional material from AP)