EL-MAHALLA EL-KUBRA, Egypt, March 6 (Reuters) - Egyptian police used force to disperse thousands of protesters who were demonstrating in solidarity with Palestinians in the Gaza Strip on Thursday, witnesses said.

Police beat and chased protesters through the streets of El-Mahalla el-Kubra, a city around 100 km (60 miles) north of Cairo, and arrested at least 20.

The protesters were mainly members of the Muslim Brotherhood, the country's most powerful opposition group, and labour union or political activists.

The protest came three days after Israel ended a northern Gaza offensive that killed more than 120 Palestinians, about half of them civilians, and aid groups said that Israel's blockade had created the worst humanitarian crisis in Gaza in more than 40 years. Israel defended the offensive as a response to rocket attacks on its civilians.

Witnesses said plain clothes police surrounded individual protesters and arrested them, taking them off to police trucks.

They added that the crackdown began with the arrest of two members of the Brotherhood, who had been riding a motorcycle and chanting pro-Palestinian slogans through a loudspeaker.

The protesters demanded their release and said they would not disperse until police let them go, and subsequently began chanting slogans attacking President Hosni Mubarak, his son Gamal, who it is widely believed is being groomed to succeed him, and Prime Minister Ahmed Nazif.

The non-violent Muslim Brotherhood usually seeks to avoid the use of anti-government slogans in its protests.

The Egyptian government has cracked down on the Brotherhood in the lead-up to local elections in April, in a bid to block it from contesting the polls, the Brotherhood says.

Police detained 26 of the Brotherhood earlier on Thursday including three leading members. A Brotherhood spokesman said more than 350 others had already been picked up in the campaign.

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. As it cannot form a political party, its members stand in elections as independents. (Writing by Aziz El-Kaissouni, editing by Mark Trevelyan)

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