Syrian opposition figures are holding simultaneous meetings in Damascus and Turkey to discuss ways of ousting President Bashar Assad.
Organizers said some 400 people are taking part in the "National Salvation Conference" in Istanbul.
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However, a small group of opposition figures, including Tammo, met in a private location in the capital and spoke by phone with the conference in Istanbul.
Addressing the Istanbul conference, Tammo said Assad had lost his legitimacy to rule and called on him to step down.
"The regime had kidnapped the entire state, and we want it back," said Haitham al-Maleh, who headed the conference.
The 80-year-old lawyer who spent years in Syrian prisons for his political activism recently left Syria, fearing for his life, and is now in Istanbul. "This regime cannot rob us of our freedom," al-Maleh said.
"What's happening in Syria is very uncertain and troubling because many of us had hoped that President Assad would make the reforms that were necessary," US Secretary of State Hillary Clinton told reporters in Turkey on Saturday. "The brutality has to stop, there must be a legitimate sincere effort with the opposition to try to make changes."
Anti-regime protests in Syria, which began in March, noted a new record Friday, with over a million protesters taking to the streets.
Meanwhile, the state-run SANA news agency says pro-government rallies took place in several cities.
Activists had urged citizens to protest against the imprisonment of hundreds of people in the government's crackdown on pro-democracy protesters.
Syrian President Assad is trying to crush a four-month old uprising against his 11-year autocratic rule. He has made promises of political reforms, but opposition groups have dismissed the offers and demanded an end to the deadly crackdown on their movement.
News agencies contributed to this report
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