ANKARA (Reuters) - Turkish security forces briefly detained 27 academics accused of terrorist propaganda, local media said, over a declaration that criticized military action in the largely Kurdish southeast and urged an end to curfews.

President Tayyip Erdogan denounced the more than 1,000 signatories of the document, including U.S. philosopher Noam Chomsky, as “dark, nefarious and brutal” in a speech after Friday prayers.

In a more ironic tone, he said those who did not want to do politics in parliament “should go dig trenches or go to the mountains” - a reference to the tactics and hideouts of the Kurdish militant group PKK.

The Turkish opposition and the U.S. ambassador to Turkey criticized prosecutors’ actions in ordering investigations and home searches of academics across the country after Erdogan criticized the signatories in a speech on Thursday.

The declaration was inspired by clashes between government forces and the PKK since a ceasefire broke down in July. The military has locked down entire districts and pounded PKK outposts in residential areas, but denies accusations that its actions have endangered and killed civilians.

The majority of Friday morning’s detentions were in the western industrial province of Kocaeli, close to Istanbul and all had been released by the evening, local media reported. The Kocaeli prosecutor’s office had no comment.

The declaration, published on Monday, accused the government of heavy-handedness in its efforts to weed out militants, who have increasingly brought their fight out of the mountains and into the towns, creating a major headache for security forces.

“The right to life, liberty, and security, and in particular the prohibition of torture and ill-treatment protected by the constitution and international conventions have been violated,” the declaration read.

“We demand that the state abandon its deliberate massacre.”

More than 2,000 lawyers signed and published online a pledge to offer free legal assistance to the academics.

“The declaration does not praise or call for hate or committing crimes,” said one law professor, asking not to be named for fear he himself would be prosecuted.

“CHILLING EFFECT”

The document has garnered support from hundreds of academics around the world, but incensed nationalists inside Turkey. The PKK, fighting since 1984 firstly for an independent Kurdistan and now for Kurdish autonomy, is considered a terrorist organization by the United States, the European Union and Turkey.

Turkey’s leading opposition party, the CHP, described the detentions as “totally lawless”.

Turkey regularly performs badly in surveys on press freedom and freedom of speech and has been heavily criticized by its western and European partners.

“While we may not agree with the opinions expressed by those academics, we are nevertheless concerned about this pressure having a chilling effect on legitimate political discourse,” U.S. Ambassador John Bass said in a statement on the embassy’s Twitter feed on Friday.

In Washington, a senior Obama administration official said he expected U.S. Vice President Joe Biden to raise the issue of the academics’ detention when he meets Erdogan and Turkish Prime Minister Ahmet Davutoglu during a Jan. 21-23 visit to Turkey.

“We do have concerns whenever any of our democratic partners which take actions which, you know, I think can be perceived as curtailing freedom of speech or activities by civil society groups,” said the official, briefing reporters on condition of anonymity.

In an apparent sign of U.S. concern, Biden is due to meet with Turkish civil society leaders a day before he holds talks with Erdogan and Davutoglu.

Meanwhile, clashes continued between government forces and PKK fighters, with five militants and a policeman killed in the eastern town of Siirt, security sources told Reuters. The army said in a statement that 19 PKK militants had been killed on Thursday in three southeastern provinces under curfew.

The southeastern towns of Cizre and Silopi, bordering Iraq and Syria, and the historical district of Sur in Diyarbakir province have been under curfew for weeks. In Diyarbakir’s Cinar district, a truck bomb attack by the PKK killed six people on Thursday including civilians, security sources said.

Selahattin Demirtas, co-chair of Peoples’ Democratic Party (HDP) said he did not know who had carried out the attack in Cinar. “But those who did this should come forward and apologize to the public for massacring civilians, baby and children,” he added.