"My sister Dr Nooshin Ebadi was arrested at 9pm [16:30 GMT] on December 28 by four intelligence agents at her home and sent to prison," Ebadi said in a statement carried by the opposition Rahesabz website.
"I am not aware of the place of her detention or the reason for her arrest."
MPs accused the protesters, who poured onto the streets in the latest display of anger at the disputed presidential election in June, of being "anti-religion" and "counter-revolutionaries".
"Over the past six months, violence has been used, a lot of people have been arrested, tens of people have been killed, but yet you don't see any decrease in the level of demonstrations"
Muhammad Sahimi, University of Southern California
Ali Larijani, the parliamentary speaker, said the legislative body "wants the judiciary and intelligence bodies to arrest those who insult religion and impose the maximum punishment on them without reservation".
He also said parliament condemned "disgusting comments" of foreign governments after Sunday's unrest.
Barack Obama, the US president, has called on Iran to release those rounded up in Iran's crackdown.
"We call upon the Iranian government to abide by the international obligations that it has to respect the rights of its own people.
"We call for the immediate release of all who have been unjustly detained within Iran," he said in Hawaii on Monday.
Larijani responded on Tuesday saying Obama should be more concerned about "the behaviour of his troops in Guantanamo, Abu Ghraib, Afghanistan and Iraq".
"Your admiration for the opposition movement protesters will ruin your reputation and will also reveal where the movement of this anti-religious group is linked to," he said, reading from a statement prepared on behalf of the Iranian parliament.
Larijani's remarks came as thousands of Iranians took to the streets of Tehran and Isfahan to show their support for the government of Mahmoud Ahmadinejad, the president.
The Iranian foreign ministry has accused Western countries of fomenting the violence that left at least eight people dead.
Ramin Mehmanparast, the foreign ministry's spokesman, said on Tuesday that countries such as the US and UK had "miscalculated" by siding with the opposition.
Clashes broke out after police used teargas, batons, and eventually live rounds to try to disperse thousand of protesters.
Muhammad Sahimi, an Iran expert at the University of Southern California in the US, said the government's crackdown was unlikely to stop the opposition.
"If they were going to be cowed, they should have been by now," he told Al Jazeera.
"Over the past six months, violence has been used, a lot of people have been arrested, tens of people have been killed, but yet you don't see any decrease in the level of demonstrations," he said.
Sahimi said that as the government sought to suppress the movement by force, support for the opposition instead grew, expanding across the country.
"The demands have gone way beyond cancellation of elections, and now people are demanding fundamental change in the system" of government, he told Al Jazeera.
"The goal right now, is at the minimum, to weaken the position of [Iran's ] supreme leader, to make him sort of a figure head ... if not outright elimination of the supreme leader, and the writing of a new constitution."