The Brotherhood, fielding independent candidates because the authorities refuse to let them form a party, has shaken up Egyptian politics by winning 76 of the 444 elected seats in parliament, two-thirds of the way through the process.
The first day of voting for the final 136 seats is on Thursday.
The Islamists are contesting 49 places.
"The authorities have resorted to these detentions because most of the means they have used in previous stages have not worked," said deputy Brotherhood leader Khairat el-Shater.
Shater, quoted on the Brotherhood website, said practices such as thuggery, preventing Brotherhood supporters from voting and the mass registration of voters resident in other constituencies - abuses corroborated by independent monitoring organisations - had not stopped Brotherhood victories.
Most of the new detentions were in the provinces which will vote on Thursday for the remaining seats, said Brotherhood spokesman Mohamed Osama on Wednesday.
In the last two weeks alone police have rounded up more than 1600 Brotherhood members in connection with the elections but many of those have been released, the spokesman added.
Brotherhood members have
already won 76 of 444 seats
The ruling National Democratic Party (NDP) will retain a comfortable majority but the Brotherhood contingent will be the largest opposition group in parliament since the overthrow of the monarchy in 1952.
A Brotherhood statement on arrests in Dakahlia province in the Nile Delta said the arrests were intended to thwart the Islamic group's election preparations.
The people detained include representatives of the candidates and those running their election campaigns, it added.
"The Interior Ministry alleges that these people of social and political weight ... intended to corrupt the electoral process and that swords, knives and clubs were found in their possession," it said.
An Interior Ministry spokesman said he had no information on the detentions.
The authorities routinely round up large numbers of Brotherhood activists before elections to undermine the Islamist group's attempts to get out the vote.
But arrests before the second stage of voting earlier this month did not prevent the Brotherhood winning 42 seats, on top of 34 seats in the first stage.
Al-Aryan suggests Brotherhood
should have civil and charity wing
Government officials have not shown any signs of alarm at the Brotherhood's gains, but since the initial victories the security forces and NDP activists have tried to make life harder for the Islamists in many constituencies.
One cabinet minister, speaking in private, said the vote for the Brotherhood was natural after the first contested presidential elections in September, won by President Hosni Mubarak with 89% of the vote.
He said that that on top of those who voted against Mubarak the Brotherhood has attracted a substantial protest vote from people unhappy with the NDP.
The United States has prodded the Egyptian government to ensure the elections are fair but it has not condemned measures taken against the Brotherhood, which opposes US policies.
Message to US
Presidential runner-up Ayman Nur, who was denied victory in his own Cairo stronghold in the first round, said the regime had chosen to crush the secular opposition.
"The rise of the Brotherhood is of course the reflection of the movement's strength but the regime had decided to use it as a message to the United States," he told AFP.
Ayman Nur: Brotherhood's rise is
a reflection of their strength
"They are saying: 'You will get Islamists in power if we leave'.
It was designed to scare the West but they hadn't quite counted on such scores," said the Ghad party leader.
The Brotherhood has conducted a well-crafted campaign under the slogan "Islam is the solution" but its critics have demanded the movement spell out its political programme.
In Wednesday's London-based Arab daily Al-Hayat, Brotherhood spokesman Issam al-Aryan suggested splitting the movement into a "civil" political party and a religious-based charity network.
Yet the movement's various currents and generations hold differing views on the path to follow.
Its newfound political strength will likely boost its case for legalisation as a political party, although the ruling party and Washington have consistently ruled out such a move.