CAIRO, Feb 8 (Reuters) - Egyptian security forces detained senior members of the Muslim Brotherhood on Monday, sources close to the group said, calling it a crackdown on Egypt's biggest opposition group before elections this year.

The Brotherhood, though banned, won a fifth of the seats parliament in 2005 when members ran as independents but since then authorities have squeezed the Islamist group out of mainstream politics.

Analysts expect the group's support to shrink at the parliamentary elections in the second half of this year.

Security forces raided the homes of deputy leader Mahmoud Ezzat, a member of the governing body Essam al-Erian and other senior members early on Monday and detained them, a lawyer said.

These 14 arrests net a number of leading members following the internal elections and ahead of parliamentary elections, lawyer Abdel-Moneim Abdel-Maksoud 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.

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, said Mohamed el-Katatni, a member of group's guidance bureau.

Mohammed Badie, who was picked as leader in January, told Reuters the group was committed to vigorously campaigning in the parliamentary election but said a state crackdown would likely prevent a repeat of the success achieved in 2005.

The government of President Hosni Mubarak, whose predecessor was gunned down by Islamic militants, is wary of any group with Islamist leanings, including the Brotherhood which long ago renounced violence and insists it seeks peaceful reform.

Security forces detained 20 other members of the Brotherhood on Saturday and Sunday. (Writing by Marwa Awad; Editing by Jon Hemming)