Essam Derbala, a leading member of Al-Gamaa Al-Islamiya's Building and Development Party (BDP), was arrested late Tuesday in Upper Egypt's Qena.
Derbala's defence team said that the prosecution is questioning Derbala on his membership in the banned Muslim Brotherhood-led National Alliance to Support Legitimacy (NASL).
The Islamist group has played a key role in the founding of the NASL in June 2013 to rally around then be-sieged Islamist president Mohamed Morsi.
After the ousting of Morsi in July 2013, the Islamist alliance has mobilised for the reinstatement of Morsi and the 2012 constitution.
Some critics have accused Al-Gamaa Al-Islamiya, along with the Brotherhood, of involvement in terrorist attacks over the past two years across the country.
However, Derbala, who is a senior member of the executive board of the BDP, has repeatedly denied accusations that Al-Gamaa Al-Islamiya and the Muslim Brotherhood share identical aims, saying that Al-Gamaa adopts "moderate revolutionary Islamic" beliefs.
Since late 2013, Derbala, who has maintained since Morsi's ouster that his group shares the Brotherhood's stance of opposition to the post 30 June government, has declared that his group does not necessarily seek, as the Brotherhood do, the reinstatement of ousted president Morsi.
The ultraconservative group, undergoing internal divisions since late 2014, has been considering exiting the pro-Morsi alliance.
Morsi and many brotherhood leaders as well as other Islamists are currently being tried on charges including incitement of violence, murder and otther criminal charges..
Al-Gamaa Al-Islamiya is responsible for the assassination of late Egyptian president Anwar El-Sadat in 1981, as well as the murder of 58 tourists and four Egyptians in an attack in Luxor in 1997.
However, the group underwent a process of 'revisionism' in the early 1990s, renouncing the use of violence to achieve political aims.
In the aftermath of the January revolution, Al-Gamaa formed a legal political party, the BDP, and ran in parliamentary elections.