Egyptian security forces have detained three of the Muslim Brotherhood's senior leaders and at least 10 other members.
Mahmoud Ezzat, the Brotherhood's newly elected deputy leader, and two other members of the group's guidance council - identified as Essam el-Erian and Abdul-Rahman el-Bir - were arrested in the early hours of Monday.
A police official, speaking on condition of anonymity because he is not allowed to speak to the media, said they were arrested for engaging in banned political activity.
Sources close to the Brotherhood said the arrests were a crackdown on Egypt's largest opposition group ahead of elections this year.
Though banned, the Brotherhood won a fifth of the seats parliament in 2005 when its members ran as independents, but since then authorities have squeezed the group out of mainstream politics.
Al Jazeera's Amr el-Kahky reporting from the Egyptian capital of Cairo, said: "Even though the Brotherhood has been banned since the 1950s, its members still run in elections.
The latest arrests send a message of no tolerance from the government just as parliamentary elections in the country are about to take place. The government will always deal with them as a banned group."
Gamal Nassar, the leader of the Muslim Brotherhood in Egypt, told Al Jazeera: "The arrests will definitely affect Egypt’s image, both internally and externally.
"The Egyptian regime must review such practice because the Muslim Brotherhood has a presence on the streets of Egypt, it gains legitimacy from the people and has 86 seats in the parliament," he said.
The Brotherhood is widely seen as the only opposition group capable of mobilising thousands of disciplined supporters in protest against the government, but its members are regularly rounded up before votes for national or local bodies.
Mohamed el-Katatni, a member of group's guidance bureau, said: "This is part of the state's campaign against the group. The group is now getting ready for parliamentary elections and this campaign is to stem such activities."
Security forces detained 20 other members of the Brotherhood on Saturday and Sunday.
Supporters of Egypt's Muslim Brotherhood have staged a number of protests in recent years demanding the government to release its members.