Uganda's biggest opposition party on Wednesday filed a case in the constitutional court to nullify a newly enacted law that reinstated the right of military tribunals to try civilians, a lawyer and party official said.

The Supreme Court proscribed the prosecution of civilians in the tribunals in January, saying they lacked legal competence to conduct criminal trials in a fair and impartial manner.

Local and international rights campaigners have long accused Uganda's government of using military courts to pursue political opponents of President Yoweri Museveni, 80, who has been in power for almost 40 years.

His aides have denied this, saying only civilians who use guns for political violence are prosecuted in such tribunals.

George Musisi, lawyer for the opposition National Unity Platform (NUP), said the party had filed a petition to quash the new law which parliament passed in May and Museveni signed a month later.

"They violated all the processes that they should follow while passing laws," NUP's secretary general Lewis Rubongoya told reporters outside court, citing a lack of sufficient public consultation.

Many supporters and leaders of NUP, including pop star-turned politician Bobi Wine who is preparing to challenge Museveni in next year's election, have previously been charged or prosecuted by military tribunals.

After Museveni signed the law, the then-army spokesperson, Chris Magezi praised it, saying it would "deter the formation of militant political groups" while Uganda's military chief Muhoozi Kainerugaba, who is also Museveni's son, congratulated lawmakers for passing the law.

The Supreme Court verdict forced authorities to transfer the military trial of a major opposition figure, Kizza Besigye, for alleged offences including treason, to a civilian court.

Besigye is still in detention, almost nine months after he was detained and has been repeatedly denied bail.