Greece has rushed in extra squads of riot police to Lesbos amid warnings of potentially explosive tensions on the island following clashes between security forces and thousands of migrants and refugees.

As hundreds of mainly Afghan asylum seekers converged on Mytilene, the local capital, on Tuesday to protest against conditions in the island’s vastly overcrowded camp, the government ordered in the reinforcements.

“It’s a powder keg ready to explode,” Kostas Moutzouris, the Northern Aegean regional governor, told Skai TV. “Yesterday I was not just afraid, I was shaking [at the thought] of where the situation could go,” he said, referring to Monday’s violent protests in which riot police fired teargas to disperse the crowd.

“Things have reached a limit, especially on Lesbos where we have 25,000 migrants.”

At Moutzouris’s request, the government agreed to dispatch a further two riot police units in addition to extra squads that were ordered in on Sunday. The reinforcements were expected to strengthen patrols around the sprawling migrant camp outside the hilltop village of Moria once they arrive late on Tuesday.

The move came as the United Nations refugee agency called on authorities to urgently move thousands of refugees from Lesbos, Samos, Chios, Leros and Kos, the islands on the frontline of what has been a surge in the number of arrivals by sea from Turkey.

Targeted with renewed vigour by human traffickers ferrying boatloads of desperate people from the Turkish coast, more than 74,000 asylum seekers reached Greece last year, more than any other place in Europe. More than half – 42,000 – are trapped on the islands in facilities designed to house no more than 5,400. On Samos, 7,000 people – the equivalent of the island’s population – are crammed into a camp built to host fewer than 700. Aid groups have warned of a humanitarian crisis in the making.

“A third are children and seven out of 10 of them are under the age of 12,” said Boris Cheshirkov, a spokesperson for UNHCR Greece, adding that conditions in the facilities had deteriorated sharply since the summer.

“There are also pregnant women, elderly people, survivors of torture and others whose physical and mental condition is impaired. The government must urgently implement its plan to move people to the mainland, improve conditions and enforce a fast and fair asylum procedure.”

It was crucial that other regions in Greece showed solidarity towards the islands, which have borne the brunt of the refugee crisis and the EU also opened up the relocation process, he said.

Vowing to decongest the islands, Athens’ centre-right administration had promised to transfer 20,000 people last year but with arrivals far outpacing departures the goal has not been met. Faced with a backlog of close to 90,000 asylum requests, it has also pledged to step up deportations and create “closed” camps on the outposts, a move that has infuriated locals and increased tensions on the islands.