They include the heads of three opposition parties, who were detained on Tuesday and held for a day before being released, said Hailu Kebede of the Salsay Weyane Tigray (SaWeT) party.

The arrests took place as the leaders and party activists drove around in vehicles equipped with loudspeakers urging people to join a protest on Thursday, he told AFP.

The political challenge to the TPLF -- the Tigray People's Liberation Front (TPLF) -- comes as the region emerges from a bloody two-year war.

The conflict began when Ethiopia's federal government ordered an offensive against the TPLF, a former guerrilla group that dominated Ethiopia's complex politics for 27 years.

In November 2022, the TPLF and the federal government signed a peace deal that brought the curtain down on a conflict that inflicted a huge toll in lives and damage.

A senior TPLF official has been appointed head of the region's interim government, which faces mountainous challenges ranging from distributing aid to restoring essential services.

In the post-deal turbulence, three Tigrayan parties -- the SaWeT, Baytona and the Tigray Independence Party last month set up a coalition called the Alliance for Radical Change.

Thursday's demonstration in the regional capital Mekele was called to highlight accusations against the TPLF of incompetence and autocracy.

The TPLF-controlled municipal authorities have said they are refusing to authorise the protest.

They say there is a lack of available police ahead of the Ethiopian New Year festivities on September 12.

But, said Hailu, "The TPLF... does not want alternative voices to be heard."

"They want to control everything. They don't want their authority to be challenged, they don't want the international community to see an alternative in Tigray."

"If you want to hold a peaceful demonstration, you just have to inform the authorities," he added.

"We don't need any authorisation... that's the law of this country as well as international practices."

Mekele's deputy mayor, Elias Kahsay, said on Facebook on Wednesday that the protest risked spiralling into a "riot" and called on the public to oppose it.

A spokesman for the Tigray's interim administration did not respond to an AFP request for comment.