The reported raids are part of a broad offensive by Iran's leadership to intimidate anti-government demonstrators from trying to disrupt state-backed celebrations Thursday of the 1979 overthrow of the pro-Western monarchy.
The New York Times reports that in addition to opposition activists the government has also imprisoned photographers, artists, children's rights advocates, women's rights activists, students and journalists.
Reporters Without Borders said that Iranian intelligence has arrested eight journalists within the past few days, bringing the total number of reporters detained to 65, more than in any other country.
Iranian authorities are desperate to show the upper hand on the most important day of the nation's political calendar. But the high-profile events - including a huge gathering in Tehran's Azadi Square and other places across Iran - offers a chance for opposition groups to make another powerful statement of their resolve.
Anti-government Web sites and blogs have called for a major turnout and urged marches to display green emblems or clothes - the color adopted by the anti-government movement since June's disputed presidential election.
The opposition leaders have promised to join street rallies, including the Green movement founder and former presidential candidate Mir Hossein Mousavi.
Iranian officials, however, have warned that any protests will be immediately crushed by security forces. At least eight people were killed in clashes during the last major opposition marches in late December.
The Iranian police chief, Gen. Ismail Ahmadi Moghaddam, said Wednesday that "a number" of suspected opposition supporters have been arrested in recent raids.
He gave no further details on the scope or timing of the raids, according to a report by the semiofficial Fars news agency. But he claimed some of those in custody were involved in planning demonstrations.
Some rights groups outside Iran have claimed hundreds of people have been detained in sweeps targeting suspected opposition backers.
The Fars agency - which is linked to Iran's powerful Revolutionary Guard - also quoted Iran's deputy police chief, Ahmad Reza Radan, as saying that security forces will come down hard on any displays of dissent.
"We won't allow any space to seditionists," he said.
Opposition groups did not appear deterred.
Web sites included detailed instructions on possible protest routes through Tehran and even offered detailed suggestions such as bringing whistles to drown out pro-government messages on loudspeakers throughout the city.
"All together let's keep our identity and join the rally," said a statement from Mousavi on his Web site - in an apparent reference to showing the colors of the Green movement.
In recent months, the opposition has built its street protest strategy around days of important political or religious significance in attempts to embarrass authorities. The tone of the rallies, however, has shifted from outrage over alleged fraud in President Mahmoud Ahmadinejad's re-election to wider calls against the entire Islamic system, including Supreme Leader Ayatollah Ali Khamenei.
The last large-scale marches - held to coincide with a Shiite holy day in late December - brought the most violent battles with security riots since shortly after the June 12 election.
In Geneva, a senior U.S. official said he hopes a U.N. debate next week on Iran will include probes into arrests of opposition leaders and alleged abuses of detainees.
John Limbert, who was among dozens of Americans held captive in Iran in 1979-1980, urged for a wide-ranging discussion about Iran's human rights situation by the U.N. Human Rights Council.
People in Iran have been "gassed, arrested, beaten up and shot" since its disputed presidential election in June, Limbert told reporters. "The U.S. and the international community can bear witness to what is going on there, and can speak a simple truth."
Iran's U.N. mission in Geneva declined to comment on Limbert's statements.