Gambian police arrested 137 people and more than two dozen were injured as protests calling for President Adama Barrow to honour a pledge to step down after three years in office turned violent for the first time, the government said.

Barrow came to power after a 2016 election, ending 22 years of authoritarian rule by Yahya Jammeh. But he has reneged on his campaign promise to step down by this month, saying the constitution requires him to serve out a full five-year term.

In response, a movement called "Three Years Jotna" - which means "enough" in the local Wolof language - began protesting last month to demand his departure.

On Sunday, police intervened when protesters deviated from the planned route on the outskirts of the capital Banjul in order to march toward the city centre, the government said.

In a statement late on Sunday, government spokesman Ebrima Sankareh said the protesters had stormed a police barricade and chanted that they planned to unseat Barrow.

"The protesters became riotous and violent by obstructing roads and burning tyres and logs on the highway as well as setting up fires in nearby bushes and on government wetland," Sankareh said.

Eighteen police officers and seven civilians were injured, Sankareh said, adding some of those among the 137 arrested were executive members of Three Years Jotna.

The government also decided to ban Three Years Jotna, Sankareh said, calling it "a subversive, violent and illegal movement", and suspended two radio stations it accused of inciting violence during demonstrations.

Opposition leaders could not be immediately reached for comment.

After winning plaudits at the beginning of his tenure for committing to respect rights and establish a truth commission to investigate abuses under Jammeh, Barrow is facing multiple challenges.

Hundreds of Jammeh's supporters demonstrated earlier this month to demand the former president be allowed to return to Gambia from exile in Equatorial Guinea. Jammeh fled there in January 2017 under military pressure from West African countries to respect his election loss to Barrow.

The government has said Jammeh will be arrested if he returns to Gambia for killings, torture and other abuses allegedly committed by his security forces. Jammeh denies those allegations.

Barrow also faces a weak economy, hobbled by massive debts incurred by Jammeh's government.