The wife of al-Shaer told Al Jazeera that Israeli occupation forces stormed their home at dawn, placed her husband under arrest and took him to an undisclosed location.
'Failed' Shalit deal
The Israeli military said in a statement: "These men have been the leaders of the ongoing effort to restore the administrative branch of the Hamas terror organisation in the region, while attempting to strengthen the power and influence of Hamas."
Thousands of Palestinians are held in Israeli jails.
The latest detentions are being seen as an effort to pressure Hamas to release an Israeli soldier captured by Hamas-linked fighters near the Gaza border in June 2006.
Egyptian efforts to mediate the release of of the soldier, currently being held in the Gaza Strip, in return for the release of hundreds of Palestinians, collapsed this week.
Mahmoud Musleh, a Hamas politician, told the Reuters news agency: "These arrests are an angry reaction by Israel because of the failure of the [Gilad] Shalit deal.
"These arrests are an angry reaction by Israel because of the failure of the Shalit deal"
"This won't do Israel any good."
An Israeli military spokesman denied the detentions were connected.
Ehud Olmert, the outgoing Israeli prime minister, had hoped to secure the release of the soldier before leaving office.
Salam Fayyad, the outgoing Palestinian prime minister, described the arrests as repressive.
"Dr Fayyad considers these measures to be repressive, unjustifiable and rejected by the Palestinian Authority as they only contribute to increasing tensions and instability," his statement read.
Israeli arrests are part of daily incursions and raids in the villages and towns of the West Bank.
Hamas has been demanding the release of more than 400 Palestinian prisoners.