Tens of thousands of people appeared on Tehran’s streets today, again, despite Ayatollah Khamenei’s clear warning that they’d be met with force and violence.

According to reports from various quarters, the demonstrators have added a new chant to their repertoire: “Death to Khamenei!” If so, another red line — and an extremely explosive one — has been crossed.

Thousands of troops from the Revolutionary Guards, the police, and the Basij paramilitary force — the mosque-based, devout followers of Khamenei — blockaded Revolution Square in Tehran today, the proposed site of the main opposition gathering, and they used tear gas, metal batons, and water cannons to keep people out of the square.

It’s unclear whether the show of force will quell the protests. The energy is there. Video from Tehran overnight showed that the skies over the city echoes last night with cries of “God is Great!”, in what has become the movement’s call to arms. (It’s a direct echo of the anti-Shah revolution in 1979.) According to the Guardian, the regime has managed to stall the protests, and it reports that both Mir Hossein Mousavi and Medhi Karrubi, the reformist candidates on June 12, are wavering about open defiance of a regime bent on violence:

The momentum of Iran’s “green revolution” – triggered by allegations of electoral theft earlier this month – appeared to stall yesterday, as thousands of plain clothes and uniformed security officials swamped Tehran, using tear gas and water cannon on a hard core of about 3,000 demonstrators.

The paper reports a “climate of fear growing in Iran,” adding:

In the first indications that the extraordinary week of protests might be coming to an end, Karoubi’s Etemad-e Melli party said plans for their participation in the rally yesterday had been scrapped for lack of a permit. “Because of not obtaining permission, the rally today has been cancelled,” a party spokesman said yesterday. An ally of Mousavi said the politician had urged his supporters not to march on Saturday or Sunday.

But other reports say that Mousavi has called for a general strike, and that he has said that he is prepared for martyrdom:

Opposition leader Mirhossein Mousavi said he was “ready for martyrdom,” according to an ally, in leading protests that have shaken the Islamic Republic and brought warnings of bloodshed from Iran’s Supreme Leader.

Mousavi also called on Saturday for a national strike if he is arrested, a witness said.

The showdown is building.

Here’s President Obama’s latest:

The Iranian government must understand that the world is watching. We mourn each and every innocent life that is lost. We call on the Iranian government to stop all violent and unjust actions against its own people. The universal rights to assembly and free speech must be respected, and the United States stands with all who seek to exercise those rights.

As I said in Cairo, suppressing ideas never succeeds in making them go away. The Iranian people will ultimately judge the actions of their own government. If the Iranian government seeks the respect of the international community, it must respect the dignity of its own people and govern through consent, not coercion.

Martin Luther King once said – “The arc of the moral universe is long, but it bends toward justice.” I believe that. The international community believes that. And right now, we are bearing witness to the Iranian peoples’ belief in that truth, and we will continue to bear witness.