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.
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 and the EU as well as 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," US Ambassador John Bass said in a statement on the embassy's Twitter feed on Friday.
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.