Turkish police attacked marchers celebrating Republic Day today, firing tear gas and water cannon at the crowds.
The government denied authorisation for the traditional march to the mausoleum of the republic's founder Mustafa Kemal Ataturk, citing security reasons.
But tens of thousands showed up anyway and their numbers would have been even greater had the authorities not prevented more than 100 coachloads of people from entering the capital.
In recent years Republic Day has become a rallying point for protesters who believe the government of Prime Minister Recep Tayyip Erdogan is abandoning Turkey's secular tradition and leaning towards Islamist government.
The celebrations revealed a divided nation with a military parade taking place in one part of Ankara attended by President Abdullah Gul and Mr Erdogan, while a mass rally organised by dozens of civil society organisations was held at the mausoleum just a few streets away.
Demonstrators waved Turkish flags and chanted: "We are the soldiers of Mustafa Kemal" and "Turkey is secular and will remain secular."
But police attacked a group "shouting anti-government slogans" with pepper-spray before turning water cannon on the main crowds.
Marchers also accused the authorities of using tear gas.
"On the 89th anniversary of our great leader's foundation of the republic, in the very place he made the declaration, they use gas against the people. It is so very sad," said opposition activist Alper Kafa.
Police appeared to back down after the initial attack due to the size of the rally and lifted their barricades, though more than 3,000 officers continued to monitor it.
Opposition leader Kemal Kilicdaroglu of the Republican People's Party, who addressed the banned rally, told Turkish media: "These people had only Turkish flags in their hands, but the state had police and pepper spray.
"Why? What can be as natural as celebrating Republic Day?"
Activists began a sit-in outside the European Parliament in Brussels today in support of Kurdish prisoners on hunger strike in Turkey.
More than 680 prisoners in 58 prisons have gone on hunger strike to demand Turkish recognition of the Kurdish language in schools and courts, and for an end to the solitary confinement of Kurdish nationalist leader Abdullah Ocalan.
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