Ayatollah Ali Khamenei, who backs the re-election of President Ahmadinejad, is due to speak at Friday prayers.
The authorities have been laying on bus services and urging people to come to hear Mr Khamenei speak.
The opposition has called off a rally planned for later, after four straight days of demonstrations.
The BBC's Jon Leyne in Tehran says the ayatollah's address is going to be a crucial moment in the crisis.
It will be the first public appearance by the supreme leader since he gave his unequivocal backing to the election result and President Ahmadinejad.
Everyone in Iran will be watching to see whether Ayatollah Khamenei offers a message of conciliation to the opposition, or signals the major crackdown many people fear.
The ayatollah will deliver a sermon at the University of Tehran - scene of several clashes between police and students in recent days.
Under the republic's constitution, the supreme leader has unfettered power to run the country and shape policy.
His anticipated appearance follows days of rallies by backers of presidential rival Mir Hossein Mousavi, who believe the vote was fixed.
On Thursday, more than 100,000 people attended a "day of mourning" rally in Tehran, called by Mr Mousavi, to commemorate up to eight people killed during protests earlier in the week.
A protest planned for Friday has been called off, but the opposition says it will return to the streets on Saturday.
Government clampdown
The rallying cry of the protesters has been "death to the dictator" - a chant surely directed at Ayatollah Khamenei, our correspondent says.
Whether the protesters understand it or not, he says, they are implicitly challenging the whole system.
Hundreds of thousands of Mr Mousavi's supporters have taken to the streets this week in several mass rallies - the biggest protests in the Islamic republic's 30-year history.
Those who died were killed when the protesters surrounded a compound occupied by a militia that backs the government on Monday.
And the unrest has spurred the authorities to clamp down on dissent by blocking websites, restricting journalists and arresting dozens - possibly hundreds - of activists they regard as opponents.
Mr Mousavi and two other candidates in the election have made more than 600 complaints to the Guardian Council - the main electoral authority.
The objections include a shortage of ballot papers, voters being pressurised to support a particular candidate and the barring of candidates' representatives from polling stations.
The council has invited the three to a meeting to discuss their objections on Saturday.
Mr Ahmadinejad defended his election win in a televised address on Thursday - which correspondents say is a sign he is taking the protesters more seriously.
"In this election, victory belonged to 70 million Iranians and the 40 million who took part in voting. Everyone is a winner," he said.
