Turkey will charge 18 more people over recent mass anti-government protests, including with alleged membership in a "terror" group, lawyers and media said on Thursday.

The 18 were among the 90 members of the small leftist group, the Socialist Party of the Oppressed (ESP), who were arrested over the demonstrations that have presented Prime Minister Recep Tayyip Erdogan with the biggest challenge of his decade-long rule.

They will face charges of "membership in a terrorist organisation" and "damaging public property" and will face several years in prison if convicted, NTV reported.

The group is among several that has been active in the anti-government protests and its members were detained in a police raid on Tuesday, the Contemporary Lawyers Association (CHD) said.

They bring to at least 24 the number of people who will face charges over the demonstrations that infuriated Erdogan and that have earned Turkey criticism from the West. Most observers expect the number of those charged to rise.

Turkey has taken a tough stance against the tens of thousands of demonstrators who have been protesting since May 31 against Erdogan's Islamic-rooted government, seen as increasingly authoritarian and conservative.

Police have used tear gas, water cannon and rubber bullets to quell the demos, leaving thousands injured and four dead.

Doctor unions suspect heavy use of tear gas might have led to the death of two more people, whose bodies are still being examined.

Protests in Istanbul have evolved into passive demonstrations and political workshops but hundreds continue to clash sporadically with the police elsewhere in Turkey, including in the capital Ankara.