TEHRAN, Iran - In the final hours of Iran's fierce election campaign, the top pro-reform challenger to President Mahmoud Ahmadinejad got a sharp warning yesterday that authorities would crush any attempt at a popular "revolution" inspired by the huge opposition rallies calling for more freedoms.

The threat by an official of the powerful Revolutionary Guard reflected the tense atmosphere surrounding tomorrow's up-for-grabs election. It also marked a sharp escalation by the ruling clerics against the youth-driven campaign of Mir Hossein Mousavi and its hopes of an underdog victory.

The Revolutionary Guard is one of the pillars of the Islamic establishment and controls large military forces as well as a nationwide network of militia volunteers.

The message from the Guard's political chief, Yadollah Javani, appeared to carry twin purposes - to rattle Mousavi's backers and to warn that it would not tolerate the formation of a postelection political force under the banner of Mousavi's "green movement" - the signature color of his campaign.

In a statement on the Guard's Web site, Javani drew parallels between Mousavi's campaign and the "velvet revolution" that led to the 1989 ouster of the communist government in what was then Czechoslovakia.

Calling the use of a color a "sign of kicking off a velvet revolution project in the presidential elections," Javani vowed that any "attempt for velvet revolution will be nipped in the bud."

He also accused the reformists of planning to claim vote rigging and provoke street violence if Mousavi loses.

Ahmadinejad is believed to have wide support in the Revolutionary Guard and among Iran's ruling clerics, though neither have given public endorsements in a presidential race that has seen the sudden and unexpected rise of Mousavi, who served as prime minister in the 1980s.

Mousavi's proposals include creating an international consortium to oversee uranium enrichment in Iran. The West fears Iran could eventually seek nuclear weapons; Iran says it wants only peaceful reactors for electricity.

Even after the official end of campaigning at midnight, tens of thousands of Mousavi supporters remained in Tehran's streets, dancing on cars, waving green flags, and passing pro-Mousavi flyers into car windows.

Ahmadinejad also drew tens of thousands of flag-waving backers, including many women in black chadors, as he claimed he was the victim of Nazi-style propaganda.

"They applied the methods of Goebbels, propaganda minister of Hitler," Ahmadinejad told supporters jamming a street.

It was a reference to Ahmadinejad's repeated claims that Mousavi has exaggerated Iran's economic problems to discredit the government.

Mousavi has dismissed his rival's charges as an attempt to whitewash the scope of Iran's problems, which include double-digit inflation and chronic unemployment.

Mousavi also criticized Ahmadinejad for hurting Iran's world reputation by questioning the Holocaust and seeking Israel's destruction.

His supporters' calls are similar to the days of reformist President Mohammad Khatami - more social freedoms, media openness, and outreach to the West.

But now there are bigger stakes, including how to respond to President Obama's offer of a dialogue after a nearly 30-year chill.

The election outcome will have little direct impact on Iran's key policies - including its nuclear program or possible talks with Washington - which are dictated by the ruling clerics. Still, the president has influence over some domestic affairs, such as the economy, and serves as Iran's top spokesman on the international stage.