They said Musa Mustafa Musa, deputy head of the newly founded al-Ghad party, was detained on Wednesday on a warrant issued by the attorney-general in connection with an investigation of the party's leader, Ayman al-Nur.
An Egyptian state security judge on January 31 ordered the detention of Nur for 45 days pending a fraud investigation.
Aljazeera reports that the arrest was ostensibly related to a case of forgery of which the detained al-Nur was a beneficiary.
Egypt's attorney-general said the detention was in connection with criminal charges involving forgery, and not for political issues.
The MP was accused of falsifying 1178 documents submitted to Egypt's political parties' committee in support of his application to register al-Ghad movement.
Nur's arrest came just days after he met with visiting former US secretary of state Madeleine Albright, who heads a US body to promote democracy.
Egyptian human-rights groups have called for the immediate release of Nur, who was elected party leader on 6 November.
They said Nur's arrest represented "a message to other Egyptian parties at a time when many segments of Egyptian society are demanding political and democratic reforms".
Husni Mubarak may seek a fifth
six-year term as president
Musa was out of the country at the time of Nur's arrest.
The party leader, who is a member of the union of Egyptian journalists, threatened to go on hunger strike from Saturday in protest against his continued detention.
He said the charges against him were politically motivated and issued the threat during a brief prison visit by a delegation from the union.
But attorney-general Mahir Abd al-Wahid said Nur was being questioned in connection with a "criminal case under the penal code" and insisted that legal procedures were being followed.
Nur was arrested outside parliament after the house lifted his immunity following a brief debate.
The Egyptian Organisation for Human Rights on Wednesday denounced a decision by the Higher Press Council to suspend the launch of al-Ghad party's mouthpiece, which had been due to appear on the streets earlier in the day.
The decision amounted to "a violation of freedom of opinion and expression", the group said.