A prominent pro-Kurdish lawyer and rights activist has been shot dead in Diyarbakir, south-eastern Turkey.
Tahir Elci was killed in a gun battle between police and unidentified gunmen. Two police officers also died.
He had been making a statement calling for an end to violence between the Turkish state and the Kurdish rebel group the PKK when he was shot dead.
Police in Istanbul fired water cannon to disperse crowds demonstrating against Mr Elci's death.
A curfew has now been imposed in the neighbourhood where Mr Elci was shot.
Mr Elci, who was head of the Diyarbakir Bar Association, had previously been detained and received death threats after saying the separatist PKK should not be regarded as a terrorist organisation.
In July violence resumed in the conflict between the army and the PKK after a ceasefire collapsed. Dozens have been killed in violent clashes since.
"We do not want guns, clashes and operations here", Mr Elci was saying before being shot.
"The moment the statement ended, the crowd was sprayed with bullets," a local official from the pro-Kurdish HDP party, Omer Tastan, told Reuters.
The US embassy in Ankara said it was "shocked" at Mr Elci's "tragic death", calling him a "a courageous defender of human rights".
The incident sparked instant protests in Istanbul, Ankara, Izmir and Diyarbakir.
A crowd on Istanbul's central Istiklal Street chanted "We are all Tahir Elci" and "You can't kill us all," accusing the state of complicity in the attack.
Government supporters said Mr Elci was killed in a PKK attack while opponents blamed Turkish nationalists, talking of a "planned assassination", says the BBC's Mark Lowen in Turkey.
A Turkish government official said in a written statement: "We are determined to shed light on today's fatal incident in Diyarbakir. An official investigation has been launched and we have assigned four inspectors to this case.
"At this time, we believe that Mr Elci may have been caught in the crossfire but we aren't ruling out the possibility that a third party directly targeted him."
President Recep Tayyip Erdogan said the incident justified the state's "fight against terrorism".