Egyptian state television broadcasts a photo of incendiary preacher Safwat Hegazy, surrounded by security forces.
Nile TV reported on Wednesday that Hegazy, who has links to the Muslim Brotherhood, was arrested while trying to flee Egypt across the Libyan border.
He was with Murad Ali, a media advisor to the Brotherhood's political party, who was also arrested.
The two are among the latest supporters of the Brotherhood to be detained as authorities here crack down on sympathizers.
Mohamed Badie, the group's leader, was taken into custody earlier this week.
Other high-ranking members have already been detained, prompting many Brotherhood supporters and officials to go into hiding.
The news comes as security forces keep a close eye on a Cairo neighbourhood where anti-government protesters had set up camp for six weeks.
The camp and a smaller sit-in were forcibly cleared out last week, resulting in the deaths of hundreds and leaving the area in ruins.
Egypt's military has been signalling its determination to crush the group and silence all protests after former president Mohamed Mursi was toppled in July.
The government has declared a month-long state of emergency and enforced a curfew from dusk to dawn.
More Muslim Brotherhood supporters arrested (1:27)
Aug. 21 - Egypt arrests two Islamist figures who were trying to flee the country across the border with Libya, state TV reports. Jessica Gray reports. ( Transcript )