CAIRO (Reuters) - Thousands of Muslim Brotherhood members demonstrated in four Egyptian provinces on Thursday to protest against the government obstructing nominations for local council elections due next month.

Brotherhood and security sources said thousands of members protested in Alexandria, Gharbia, Fayoum and Sharkia provinces on the final day of a 10-day nominating period for potential candidates to register for the April 8 ballot.

The Brotherhood, Egypt's strongest opposition group, says its members have been systematically blocked from registering to stand by bureaucratic obstruction and police harassment.

Hundreds of likely candidates have been detained, it says. Police detained 15 of the group's members on Thursday during a protest in Gharbia, Brotherhood sources said.

Brotherhood officials have said that while some of the group's members had successfully filed nomination papers, past experience suggested this was no indication they would be placed on the ballot.

The Brotherhood seeks an Islamic state through non-violent, democratic means. The government calls it a banned organisation but allows it to operate within limits, but will not let it form a political party. Members stand in elections as independents.

In the last few days, various courts have issued rulings in favour of roughly 1,000 people trying to stand, though court ruling are often not enforced.

Egyptian police have intensified a crackdown on the Brotherhood in recent weeks, detaining hundreds of the group's members since mid-February in advance of the ballot.

Egypt postponed local council elections for two years in 2006 after the Brotherhood performed better than expected in a parliamentary election in 2005. The group holds roughly a fifth of the seats in the lower house of parliament.

The White House said on Thursday that it was concerned by Egypt's campaign of arrests of members of the opposition involved in forthcoming elections.