Thousands of demonstrators massed on streets surrounding Istanbul's central Taksim Square, a venue for political unrest, while protests erupted in the capital, Ankara, and the Aegean coastal city of Izmir.
Broken glass and rocks were strewn across a main shopping street near Taksim. Primary school children ran crying from the clouds of tear gas, while tourists caught by surprise scurried to get back to luxury hotels lining the square.
The unrest reflects growing disquiet at the authoritarianism of Prime Minister Tayyip Erdogan and his Islamist-rooted Justice and Development Party (AKP).
Riot police clashed with tens of thousands of May Day protesters in Istanbul this month. There have also been protests against the government's stance on the conflict in neighbouring Syria, a tightening of restrictions on alcohol sales and warnings against public displays of affection.
"We do not have a government, we have Tayyip Erdogan. ... Even AK Party supporters are saying they have lost their mind, they are not listening to us," said Koray Caliskan, a political scientist at Bosphorus University, who attended the protest.
"This is the beginning of a summer of discontent."
The protest at Taksim's Gezi Park started late on Monday after trees were torn up under a government redevelopment plan, but has widened into a broader demonstration against Erdogan's administration. Friday's violence erupted after a dawn police raid on demonstrators who had been camped out for days.
"This isn't just about trees anymore, it's about all of the pressure we're under from this government. We're fed up, we don't like the direction the country is headed in," said 18-year-old student Mert Burge, who came to support the protesters after reading on Twitter about the police use of tear gas.
"We will stay here tonight and sleep on the street if we have to," he said.
Thousands chanting for the government to resign gathered at a park in the centre of Ankara, where police earlier fired tear gas to disperse several dozen opposition supporters trying to reach the AKP headquarters. Protesters also rallied at two locations in Izmir, according to pictures on social media.
A Turkish woman of Palestinian origin was in a critical condition after being hit by a police gas canister, hospital sources said. The 34-year-old, who doctors had earlier identified as Egyptian, was undergoing an operation after suffering a brain haemorrhage.
A total of 12 people, including a pro-Kurdish MP and a Reuters photographer, suffered trauma injuries and hundreds suffered respiratory problems due to tear gas, doctors said.
Some people were injured when a wall they were climbing collapsed as they tried to flee clouds of tear gas.
Amnesty International said it was concerned by "the use of excessive force" by the police against what had started out as a peaceful protest. Ria Oomen-Ruijten, the European parliament rapporteur on Turkey, also voiced concern.
In Washington, the State Department said it was concerned with the number of injuries and was gathering its own information on the incident.
"We believe that Turkey's long-term stability, security and prosperity is best guaranteed by upholding the fundamental freedoms of expression, assembly and association, which is what it seems these individuals were doing," State Department spokeswoman Jen Psaki told reporters.
Interior Minister Muammer Guler promised that allegations that police had used disproportionate force would be investigated.
Erdogan has overseen a transformation in Turkey during his decade in power, turning its economy from crisis-prone into Europe's fastest-growing. Per-capita income has tripled in nominal terms since his party rose to power.
He remains by far Turkey's most popular politician, and is widely viewed as its most powerful leader since Mustafa Kemal Ataturk founded the modern secular republic on the ashes of the Ottoman Empire 90 years ago.
But Erdogan brooks little dissent. Hundreds of military officers have been jailed for plotting a coup against him in recent years. Academics, journalists, politicians and others face trial on similar charges.
He has made no secret of his ambition to run for the presidency in elections next year when his term as prime minister ends, increasing opposition dismay.
"These people will not bow down to you" read one banner at the Gezi Park protest, alongside a cartoon of Erdogan wearing an Ottoman emperor's turban.
Postings on social media including Twitter, where "Occupy Gezi" - a reference to protests in New York and London last year - was a top-trending hashtag, and Facebook said similar demonstrations were planned for the next few days in other Turkish cities including Ankara, Izmir, Adana and Bursa.
"Kiss protests," in which demonstrators are urged to lock lips, had already been planned for Istanbul and Ankara this weekend after subway officials were reported to have admonished a couple for kissing in public a week ago.
Erdogan is pushing ahead with a slew of multibillion-dollar projects he sees as embodying Turkey's emergence as a major power. They include a shipping canal, a giant mosque and a third Istanbul airport billed to be one of the world's biggest.
Speaking a few miles (km) from Gezi Park at the launch on Wednesday of construction of a third bridge linking Istanbul's European and Asian shores, Erdogan vowed to pursue plans to redevelop Taksim Square.
Architects, leftist parties, academics, city planners and others have long opposed the plans, saying they lacked consultation with civic groups and would remove one of central Istanbul's few green spaces.