A crowd has attacked the headquarters of Turkey's pro-Kurdish HDP party, amid rising violence between Turkish forces and the militant Kurdish PKK group.
Pictures from the scene appeared to show the HDP building in the capital, Ankara, on fire.
Prime Minister Ahmet Davutoglu has appealed for calm.
Earlier, Turkish ground forces crossed into Iraq in pursuit of Kurdish militants for the first time since a ceasefire with the PKK two years ago.
Turkish warplanes also launched a wave of air strikes on bases of the PKK (Kurdistan Workers' Party) in northern Iraq.
Nationalists staged rallies across Turkey on Tuesday, hours after 14 police officers were killed in a suspected PKK bomb attack on a minibus in the east of the country.
The bombing came a day after another militant attack on the Turkish military killed 16 soldiers.
HDP (Peoples' Democratic Party) lawmaker Garo Paylan told Reuters news agency that hundreds of protesters had attacked the building in Ankara.
"Police are just watching, he said. "What's being broken there is our hope of living together."
HDP offices in at least six other Turkish cities were reported to have been attacked and images on social media appeared to show those in the southern resort city of Alanya also on fire.
In Istanbul, pro-government protesters again attacked the offices of the Hurriyet newspaper, smashing windows. An angry crowd had stormed the building on Sunday accusing the newspaper of misquoting President Recep Tayyip Erdogan in a TV interview.
Mr Davutoglu took to Twitter to denounce the attacks.
"It is unacceptable to damage media institutions, political party buildings and the property of our civilian citizens," he said.
"I invite all my citizens with hearts full of love for the country to calm, embrace one another, and to have confidence in the state."
HDP leaders have accused Mr Davutoglu's governing AK Party (AKP) of stoking unrest to drum up nationalist support ahead of elections on 1 November.
The HDP entered parliament for the first time in June elections and its 14% share of the vote deprived the AKP of a parliamentary majority.
The surge in violence follows the collapse of a ceasefire in July between the Turkish army and the PKK.
The truce, which began in 2013, unravelled after a suicide bombing by suspected Islamic State militants near the border with Syria. The attack led to mutual recriminations between Kurdish groups and Turkey.
Earlier on Tuesday, Turkey's Dogan news agency said two special forces units, supported by warplanes, had entered northern Iraq and attacked two groups of militants.
At least 35 rebels were killed in air raids on PKK bases at Qandil, Basyan, Avashin and Zap, Anadolu news agency reported.
President Erdogan said the PKK had suffered "serious damage" inside and outside of Turkey and was in a state of "panic".
However, militants targeted a police minibus on Tuesday as it was heading towards a border post close to the Azerbaijani exclave of Nakhchivan, killing 14 officers.
Hours later, a policeman was shot dead when suspected PKK militants opened fire on his car in the eastern state of Kunceli.
In Sunday's attack, the PKK detonated bombs near two military vehicles in the village of Daglica, close to the border with Iraq.
More than 40,000 people have died since the PKK launched its armed campaign in 1984, calling for an independent Kurdish state within Turkey.