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This has been a year of political turmoil in Iran. The spark was the re-election of President Mahmoud Ahmadinejad in June. Ahmadinejad's opponent say he won through massive vote fraud, and they've staged protests ever since.
Over the weekend, demonstrations reportedly left at least eight people dead.
Kasra Naji of the BBC’s Persian Service, who’s monitoring events from London, says these demonstrations were of a different order than previous ones.
"This time there was quite a lot of violence from security forces, and demonstrators paid in kind ... they reacted badly and violently too," said Naji. "They set police cars and motorcycles on fire, they set a police station on fire; they set fire to banks and other government departments and buildings."
Opposition websites say Iranian authorities arrested a number of opposition figures. Those detained include a former foreign minister and human rights activist.
President Obama condemned the crackdown. "Along with all free nations, the United States stands with those who seek their universal rights," he said in a statement on Monday afternoon.
"We call upon the Iranian government to abide by the international obligations that it has to respect the rights of its own people. We call for the immediate release of all who have been unjustly detained within Iran. We will continue to bear witness to the extraordinary events that have taken place there, and I'm confident that history will be on the side of those who seek justice."
The BBC's Kasra Naji believes the hardline stance of the Iranian government has radicalized the opposition party, which had initially called for a re-election. The opposition is now, according to Naji, "... thinking of basically toppling the leader of the country."
"If the vicious cycle of violence goes on, I think you can safely say that this could the beginning of the end for the regime as a whole," said Naji. "Because each time this happens, more people are convinced that this government is not a government, and they're not going to support it."
The protests have spread beyond Tehran, and are taking place in a number of other Iranian cities, and there are more demonstrations expected.
"The next big date is the anniversary of the Iranian revolution, which falls on the 11th of February, and in the meantime, I'm sure there will be other excuses for the oppositionists to come out on the streets," said Naji.
PRI's "The World" is a one-hour, weekday radio news magazine offering a mix of news, features, interviews, and music from around the globe. "The World" is a co-production of the BBC World Service, PRI and WGBH Boston.