A top leader of the banned Egyptian opposition Muslim Brotherhood has been arrested along with a number of other members, the group has said.
Protesters rally against earlier arrests of Brotherhood members
Mahmud Ezzat, who police say is the organisation's fourth-highest official, was arrested at home in Cairo at dawn on Sunday, a group spokesman said.
He said authorities wanted to stifle opposition to planned electoral reform, which the group says is cosmetic.
Scores have been held in the run-up to Wednesday's referendum on the changes.
Mr Ezzat is the secretary-general of the Brotherhood and heads its Cairo operations.
A police official told Associated Press Mr Ezzat was the highest-profile arrest from the group since 1996.
The group said he was among between 20 and 25 people arrested at dawn across the country. Police in Cairo put the arrests at 16.
Brotherhood spokesman Abdel Galil Sharnubi told the AFP news agency: "The [arrested members] are opinion-makers who have some weight on the Egyptian street and [the authorities] are seeking to cut them off ahead of the referendum."
Egypt's government is urging a "yes" vote on Wednesday, saying the amendments herald a new era of democracy.
But the opposition has called for a boycott.
It says proposed new rules allowing multi-candidate elections contain too many constraints for anyone to challenge President Hosni Mubarak.
Independent candidates need to have the backing of at least 65 members of the parliament, which is overwhelmingly dominated by the president's National Democratic Party.
The Muslim Brotherhood - the most popular opposition force in Egypt - is tolerated by the authorities but banned from operating as a political party.
It has 17 MPs sitting in parliament as independents.