The BBC's Tehran correspondent, Jon Leyne, looks at the latest events in Iran's capital after security forces prevented opposition supporters from holding a mass rally over the country's bitterly-contested presidential election result.
What do we know of Saturday's blocked demonstration?
Basically, the security forces have really tried hard to pre-empt this demonstration. They set up a big presence there before the demonstration was due to start.
What seems to be going on is a concerted government attempt, a much more organised crackdown than we've seen since the election.
They've got a strategy, or least a tactic, for dealing with these demonstrations. They have massed themselves today. They are in these running battles with the protesters but at the very least they seem to have prevented the mass of protesters from coming together and for the moment those government forces seem to have the upper hand.
Are there demonstrations elsewhere?
I haven't had any reports of that, but in previous days the demonstrators were quite good at communicating between themselves and re-assembling in other areas.
Clearly, the security forces had plenty of warning of the time and place of this demonstration so I wouldn't write off the demonstrators. They may be able to move elsewhere.
What about the reported bombing?
I have plenty of suspicion about that - it may be a bomb has gone off, but who knows, if it has, who was responsible for it.
This after all would be the perfect pretext for a crackdown and in the perfect place that you might want a bomb to go off in order to have a crackdown: the shrine of the most-revered Imam Khomeini. That would outrage government supporters.
I think we need to have plenty of scepticism about that it for the moment.
What about events in southern Tehran?
Again, we just have one eye-witness report so far of some kind of clashes between the two sides and that opposition supporters may have set fire to a campaign headquarters of President Ahmadinejad.
I think we should be very careful about whether this is happening.
Certainly the opposition has been trying very hard to portray themselves as peaceful protesters. And the government has been trying very hard to portray them as hooligans. I think we should bear that in mind the whole time.
This is a very murky situation though. There are plenty of provocateurs out there.
What might happen next?
The opposition supporters won't go away. I think we may be in for a game of cat-and-mouse.
I think the opposition supporters will want to try to re-assemble, maybe later tonight, maybe tomorrow. And of course the government will be trying to stop them.
My impression is that now the government has got its act together more, they've thought through their strategy against the demonstrators - which is clearly to try to break them up and to stop them assembling and at the same time possibly to rally their own supporters.
How quick has the government reaction been?
I've lived in this country for two years and there were no opposition demonstrations until the election campaign got going and really gathered steam in the last week.
I think the government has been so wrong-footed. They thought, even in the first day or two after the election result, they had a handle on it, that people would be frightened off.
I think they didn't imagine the numbers of people who would turn out on that massive demonstration on Monday that completely changed the game.
What's behind the change in government tactics?
Survival. It's as simple as that.
This is absolute life and death, political life and death for the supreme leader and possibly even for the Islamic Republic. This is a fight to the finish.
The supreme leader is not isolated at the moment, but if the opposition manages to mount huge demonstrations in defiance of his own explicit threats, then clearly that would threaten his position.
What does the opposition want?
They've always said they want a re-run of the election: "Where is my vote?"
But by challenging the government in such an overt way, they're actually - even if they don't know it and if they deny it - they're clearly challenging more than just one election.
But that is all they're doing - they just want to have a re-run of the election and, they say, to have a fair election.