Six senior members of the Muslim Brotherhood have been remanded in custody in Egypt, it has emerged.
The Muslim Brotherhood is the leading opposition group in Egypt
Five are wealthy businessmen who are believed to have financed the Islamist organisation. The detentions took place on Sunday.
The Brotherhood is banned in Egypt, but puts up parliamentary candidates standing as independents.
It won 26% of the seats in recent elections despite widespread reports of attempts to impede the voting process.
BBC Arab Affairs analyst Magdi Abdelhadi says the arrests are a serious escalation in tension between the government and the Brotherhood.
There are also reports of arrest warrants for the Brotherhood's businessmen in charge of the organisation's international branch.
The past month has seen the Egyptian government stepping up its campaign against the Muslim Brotherhood, our correspondent says.
But unlike previous detention rounds, this time the authorities are targeting the organisation's financial infrastructure - a network of businesses owned and run by wealthy members at home and abroad.
The latest arrests come only days after President Mubarak issued a stark warning in which he said that the Brotherhood was a threat to Egypt's national security.
Mr Mubarak has already proposed changes to the constitution that will explicitly outlaw any political organisation founded on a religious basis.
The group responded by announcing plans to relaunch itself as a political party open also to non-Muslims.
The Muslim Brotherhood is arguably the most influential Islamist organisation in the world. It is outlawed in Egypt, but has been able to field candidates for parliamentary election as independents.
Tension has been steadily rising ever since the Brotherhood made its biggest gains in parliamentary elections in Egypt last year.