Reports from Iran say several opposition activists have been arrested and international broadcasters are being jammed.
The moves come after the opposition called for a march in Tehran next week in support of the protests in Egypt.
Opposition websites say at least eight people close to opposition leader Mir Hossein Mousavi have been detained.
They include his brother-in-law, former minister Mohammad Hossein Sharifzadegan.
On Thursday, another opposition leader, Mehdi Karroubi, was placed under house arrest.
His website, Sahamnews.org, said security officials had told him that the restrictions would remain in place until after 14 February - the day of the proposed march.
The website Kaleme said Mr Sharifzadegan had been arrested at his home.
Meanwhile, the BBC Persian TV network has been jammed inside Iran.
Analysts say Tehran is trying to stop opposition groups from seizing the moment with rallies linked to the Egyptian crisis.
However, Iranian President Mahmoud Ahmadinejad has described the Egyptian protests as "a tide of awakening" and declared that the Middle East will soon be free of the US and Israel.
He was addressing a rally to mark the 32nd anniversary of the Iranian revolution.