The alleged mastermind of the killing, Erzhan Utembaev, the former head of the Senate administration, "categorically" denied the charges, according to his wife Lyudmila Ten.
Ten also said that another chief defendant, Rustam Ibragimov, denied the charges laid against him. Prosecutors believe Ibragimov killed Sarsenbaev.
Several former security officers are among the defendants.
The trial is being held in Taldy-Qorgan, some 250 kilometers north of the country's commercial capital, Almaty.
Sarsenbaev's relatives asked the court to hold its hearings in Almaty to enable relatives to attend the trial.
Sarsenbaev, a leader of the opposition party Naghyz Ak Zhol, was found dead near Almaty in February along with his driver and his bodyguard.
Sarsenbaev lived in Almaty until he was kidnapped and was apparently murdered in the Taldy-Qorgan region.
The trial was adjourned until June 19.
(AP, Interfax-Kazakhstan)