(Updates number of arrests, adds candidate's withdrawal)

By Nasser Nouri

ALEXANDRIA, Egypt, July 13 (Reuters) - Police detained 21 members of the banned Muslim Brotherhood on Sunday in northern Egypt where the Islamist group was fielding candidates for three vacant parliamentary seats, the Brotherhood said.

Security officials had earlier confirmed 17 arrests.

The Brotherhood, the strongest opposition group in the Arab country despite the ban, said police forces were blocking its campaign staff and supporters from voting in the coastal city of Alexandria and the Nile Delta province of Kafr el-Sheikh. Brotherhood candidates are running as independents.

The Interior Ministry said the charges were baseless.

The Brotherhood said on its Web site that police rounded up the 21 Islamists in Alexandria, where the group was vying with ruling National Democratic Party and other independent candidates for two seats vacant since 2005 when voting was halted after a legal challenge.

A Reuters photographer saw police detain two men who were carrying leaflets promoting a Brotherhood candidate.

Several hours into the voting in Kafr el-Sheikh, the Brotherhood's local candidate, Ragab el-Banna, announced he was pulling out of the election because of what he called police harassment and vote-rigging.

Banna told Reuters earlier he had seen glass boxes filled with ballots early on Sunday, despite a very low turnout. Witnesses and a Reuters reporter in Kafr el-Sheikh said only a trickle of voters had cast their ballots. "Go look after your land and your family," a police officer told a small crowd of Brotherhood supporters and campaign aides in front of one polling station.

An interior ministry spokesman said the electoral process was smooth and denied policemen were blocking voters.

"These are baseless allegations," he said. "The electoral process is going on with integrity and without any bias."

Sunday's arrests bring to almost 60 the number of Brotherhood members detained over the past four days.

The Brotherhood has been outlawed since 1954, but has traditionally been largely tolerated and operates openly.

An unexpectedly strong showing in a 2005 parliamentary election triggered a government crackdown against the group. Since then, several Brotherhood leaders have been prosecuted and imprisoned.

Analysts say the government wants to stop the Brotherhood from mounting a real threat to the rule of President Hosni Mubarak, in power since 1981. Mubarak's ruling party has a comfortable majority in every elected body in the country.

The Brotherhood says it wants to create an Islamic state through the ballot box. (Additional reporting by Aziz El-Kaissouni; Writing by Alaa Shahine; Editing by Caroline Drees)