The candidate, Abdel Hamed el-Senoussi, cried foul, claiming that the detainees, who include his brother Badri, should be working for him on Sunday as election monitors and campaign staff.
A police official in Qena, southern Egypt, confirmed more than 80 of el-Senoussi's supporters were detained on Friday and Saturday, but he would not give a reason. The official spoke on condition of anonymity.
El-Senoussi is standing as an independent, but is widely regarded as the candidate of the Muslim Brotherhood, a banned but tolerated organisation that is not allowed to field its own list.
El-Senoussi says he is not a Brotherhood member and points out that he is not using the campaign slogan "Islam is the solution" favoured by Brotherhood-backed candidates.
"But all sympathisers of Islamic groups are going to give me their vote," he said.
He accused the National Democratic Party of President Hosni Mubarak of instigating the arrests, saying the move was intended to "terrorise voters".
"...all sympathisers of Islamic groups are going to give me their vote"
Abdel Hamed el-Senoussi,
The Brotherhood won 34 seats in the first round of the elections on 9 November, more than doubling its representation in the outgoing parliament. The ruling NDP won 112 seats in the 454-seat parliament.
The group calls for implementing Islamic law but has long been vague about what this means. It advocates the veil for women and campaigns against perceived immorality in the media. But the group insists it represents a more moderate face of Islam than that followed in Saudi Arabia.
There are 1706 candidates competing in 72 constituencies in nine provinces in Sunday's polls.
The third round will be held on 1 December.