The prosecutor filed a case accusing the Peoples’ Democratic Party – or HDP – of colluding with a banned Kurdish militant movement.

The HDP, which is the third-largest party in Turkey’s parliament, denies any links to the militants.

The party said the case against it was “a heavy blow to democracy and law”.

“We call on all the democratic forces, the social and political opposition, and on our people to join a common fight against this political coup,” the party said on Wednesday.

In recent years many HDP lawmakers have been investigated or jailed – including its former leader, Selahattin Demirtas.

Turkish President Recep Tayyip Erdogan has long accused the HDP of being a political front for the outlawed Kurdistan Workers’ Party (PKK) militant group.

The PKK is recognised as a terrorist organisation by Turkey and its western allies, including the US and the European Union.

It has been waging an insurgency against the Turkish government since 1984, calling for increased human rights and freedoms for the Kurds within the country.

Tens of thousands of people have died in the conflict.

On Wednesday, prosecutor Bekir Sahin alleged that the HDP “was acting together with PKK terrorists and affiliated organisations, acting as an extension of such organisations”.

He said such activity threatened “to destroy the indivisibility between the state and the people”.

Mr Sahin filed the case with the Constitutional Court, which has the power to decide on the closure of political parties, according to the Hurriyet newspaper.

Critics have accused Mr Erdogan of using the courts to suppress political dissent against the government and his conservative AK Party.

The HDP has come under increasing political and legal pressure since a shaky truce between the PKK and Mr Erdogan’s government broke down in 2015.

That pressure has only intensified since a failed 2016 coup against Mr Erdogan by a mutinous faction of Turkish army officers.