Uganda's President Yoweri Museveni, one of Africa's longest-serving leaders, has signed a bill into law that removes a presidential age limit of 75 from the country's constitution.

The move allows the 73-year-old president to run for a sixth term in 2021.

Don Wanyama, senior presidential spokesperson, told reporters on Tuesday that Museveni had signed the bill on December 27.

The Ugandan parliament had passed the bill on December 20, with 317 politicians voting in favour and 97 against.

Museveni addressed the bill in his end-of-year address on Sunday, saying he saluted the members of parliament who had supported it.

Parliament "enabled us to avoid the more complicated paths that would have been required. We cannot [under-cook] the destiny of Africa", said Museveni.

The Ugandan Constitution, enacted in 1995, previously prohibited anyone younger than 35 or older than 75 from serving as president.

'Against wishes of Ugandans'

The scrapping of the presidential age limit, however, has met widespread opposition from civil rights activists, opposition politicians and religious leaders in the country.

In late September, members of parliament threw punches and clambered over benches, in a heated session discussing the plan to remove the age limits.

"This law will remain largely inconsequential because it was passed against the wishes of majority of Ugandans," Crispy Kaheru, the coordinator of the Citizens' Coalition for Electoral Democracy in Uganda, an umbrella organisation for Ugandan civil society, said in a statement.

Kaheru said there would be challenges both legally and "in the court of public opinion … in the end it is the majority of Ugandans who will have their last say on how they wish to be governed".

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Museveni has ruled Uganda since 1986 when he took power from General Tito Okello as the head of a rebel army.

He has served as president since 1996.

In 2005, he had the constitution amended to remove term limits, enabling him to stand successfully for a third, fourth and fifth consecutive term.

Museveni has presided over a government that is often accused of human rights abuses against the LGBT community, as well as residents of the north of the country where Joseph Kony and the Lord's Resistance Army, also accused of grave rights violations, have waged an armed campaign since 1987.

Uganda achieved independence from British colonial rule in 1962.

The country has not seen a peaceful transition of political power since.