BBC NEWS | Middle East | Unrest challenges Iran's republic
As demonstrations against the Iranian election result continue, the situation in Tehran is becoming unpredictable and potentially explosive.
Throughout Sunday, crowds gathered in a number of areas. Often they were not organised protests.
In traffic jams, car drivers hooted their horns in opposition to the government. Crowds stood on the pavement, chanting and showing v-signs.
In some places, the police were out in force. Some of them were in full riot gear. Others charged into action on the back of motorbikes.
They seem to have been given clear instructions not to open fire. Though occasional gunfire has been heard, mostly police have been wielding truncheons and batons in often brutal fashion.
It is difficult to get any reliable picture of the scale of the protests in Tehran, let alone the whole country.
But they spread rapidly during the evening. The cheers and chanting echoed even in customarily quiet middle-class neighbourhoods.
Many Iranians came out on to their roofs to shout "down with the dictator".
It has become a challenge not just of an election result, not just to President Mahmoud Ahmadinejad, but to the supreme leader, Ayatollah Khamenei himself.
That means it is, in effect, a challenge to the whole basis of the Islamic Republic.
For two years I have watched as young, ambitious Iranians go about their lives with growing frustration.
They feel the system stifles their aspirations. Now they feel that their intelligence and their pride has been insulted by an election result many Iranians believe is blatantly fraudulent.
And President Ahmadinejad's almost casual dismissal of their complaints just adds to the anger.
Make no mistake, President Ahmadinejad still has plenty of supporters.
They turned out in large numbers in the victory rally he held in central Tehran on Sunday afternoon.
He has focused his rhetoric on foreign governments and the international media, blaming them for stirring up the trouble.
There is a danger now that the two sides could come to blows.
And many people will fear that the government will authorise the police to open fire, if the situation slides further out of control.
Yet it is hard to see what political compromise is possible.
Mr Ahmadinejad is defiant, confident in the support of the supreme leader.
The opposition will know that the formal appeal process has minimal chance of success.
It is a situation without precedent in the 30-year history of the Islamic republic, and the outcome is impossible to predict.