According to someone present in the scene, none of the MPs belonged to Mubarak's National Democratic Party. When the independent and opposition MPs took the streets, they were joined by another 200 activists and ordinary citizens. (Correction: Nora reports two NDP MPs took part)
When barred, the MPs decided to head towards the Arab League, and met with Amr Moussa, the League's secretary general, to demand stronger action by the Arab states, and the expulsion of the Israeli and US ambassadors.
You can find pix of the demo here.