Aljazeera's correspondent in Cairo said on Sunday that 11 further members of the group were also arrested, including six from the al-Jiza governorate, three from Cairo, and one each from Alexandria and al-Gharbiya.
The Associated Press news agency, which put the number of arrests at 20, quoted police and Muslim Brotherhood officials as saying that Izzat was picked up in dawn sweeps, carried out in several provinces, along with 19 others.
Izzat, who is head of the Cairo operations of the banned but tolerated opposition movement, is the highest-profile member arrested since 1996, a police official said.
Mohammed Mahdi Akif, the supreme guide of the group, told Aljazeera on Sunday that the arrests were in response to the group's determination to continue demanding freedom of citizens and respect of the constitution and the law.
Akif vowed the arrests would not stop the group from persisting with its demands.
Mubarak has retained power
through one-candidate polls
"The arrests, made by the government to terrify us, would not change the Muslim Brotherhood group's determination to continue demanding people's right to freedom," he said.
He added that the group tried to open dialogue channels with government officials, but that its attempts had been ignored.
"No side has offered to hold talks with us or to know what we want," he added.
"Around 850 members of the group have so far been arrested. Why? We don't know," Akif said.
"Adopting this violent style is rejected by the Muslim Brotherhood group," he added.
Wednesday's referendum allows Egyptians to approve or reject changes to the constitution that will allow the nation's first multi-party presidential elections in September.
Opponents of President Hosni Mubarak, including the Muslim Brotherhood, have urged a boycott of the referendum, saying the changes will provide little more than window-dressing to the current yes/no one-candidate system.
Mubarak, who has served 24 years as Egypt's president, has always been handily reinstalled in yes/no referendums in which there were no other candidates.