MPs voted overwhelmingly last year to scrap the age limit of 75.

It meant 73-year-old President Yoweri Museveni, in power for more than 30 years, could seek re-election in 2021.

Opposition lawyers argued that the amendment was "smuggled" into law, and parliament had not followed proper procedures when adopting it.

Security has been increased at the court in the eastern city of Mbale, and some roads have been shut to prevent any trouble.

This is the first time a major constitutional petition is being heard in a regional city, reports the BBC's Patience Atuhaire from the capital, Kampala.

The hearing was believed to have been moved from Kampala to avoid drawing too much public attention to it, our reporter adds.

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The petitioners include opposition MPs, the Uganda Law Society and civil society groups.

They say the amendment is invalid because it was passed during parliamentary sessions marred by brawls and plain-clothes security operatives assaulting and dragging MPs out of the chamber.

They also argue that Mr Museveni wants to be president for life and the amendment violates a basic democratic principle, that power belongs to the people.

His allies say the amendment removes age discrimination, and was passed after widespread public consultation.

The ruling National Resistance Movement spearheaded the amendment through parliament, and Mr Museveni signed it into law.

Mr Museveni won a fifth term in 2016 in elections marred by allegations in rigging.

A two-term limit was scrapped in 2005 to allow him to stand for office again.