Egyptian police have arrested four journalists working for the broadcaster Al-Jazeera in the capital, Cairo.
They include the TV network's Cairo bureau chief Mohamed Fadel Fahmy and former BBC correspondent Peter Greste.
The interior ministry said the journalists had held illegal meetings with the Muslim Brotherhood, which was declared a terrorist group last week.
There has been a crackdown on the Islamist movement since the army ousted President Mohammed Morsi in July.
Since then, more than 1,000 pro-Morsi protesters have been killed in clashes with security forces, and thousands of Brotherhood supporters have been arrested, including the majority of its leadership.
A court will hear a case to disband the Brotherhood's political wing, the Freedom and Justice Party (FJP), on 15 February.
The journalists, who work for Al-Jazeera English, are understood to have been detained late on Sunday night.
They are Mohamed Fadel Fahmy, who holds Canadian nationality, Peter Greste, an Australian, producer Baher Mohamed and Egyptian cameraman Mohamed Fawzy, who is said to have been arrested at home.
Al-Jazeera, which is based in Qatar, has demanded their immediate release.
The interior ministry said in a statement that cameras, recordings and other material had been seized from rooms at a hotel in Cairo.
It accused the journalists of broadcasting news that were "damaging to national security".
The BBC's Bethany Bell in Cairo says Egypt's military backed government has long accused Al-Jazeera of bias, because Qatar gave financial support to the government of Mr Morsi.
Observers say Egypt's media environment has been highly charged since Mr Morsi's overthrow.
Several Islamist channels were closed down immediately after the military intervention in the summer. Al-Jazeera's Egyptian station Mubashir Misr was shut down in September.
The channel previously had its Cairo offices raided, equipment seized, and staff detained. Two of its staff - journalist Abdullah Elshamy and cameraman Mohammad Badr - arrested in July and August remain in detention, according to the Committee to Protect Journalists.
The latest arrests come after deadly clashes between police and Muslim Brotherhood supporters across Egypt.
On Friday, three people were killed - in Cairo, southern Minya province and the Nile Delta - during the violence.
Security forces detained some 265 Muslim Brotherhood supporters, officials said.
The Brotherhood was formally designated a terrorist group after the 24 December suicide bombing of a police headquarters in Nile Delta.
The government accused the movement of being behind the attack - a charge it strongly denied.
US Secretary of State John Kerry earlier called his Egyptian counterpart to express concern about the recent waves of arrests and called for an "inclusive political process".