Sharon said at the start of Sunday's cabinet meeting that relevant measures to stop the groups - who were "in essence aimed at inciting civil war" - had to be taken.
"I call upon those of you in charge of security matters, to take all necessary steps," he added.
His call comes after settler leaders warned on Friday that withdrawal plans could plunge Israel into civil war.
A group of prominent hardliners urged security forces to disobey orders to dismantle settlements.
Settler supporters turned out in a peaceful show of force in Jerusalem on Sunday night to defy Sharon.
Witnesses and police sources put the number of demonstrators at 70,000.
"(Sharon) should be ashamed of himself. We should find another prime minister immediately," said 18-year-old Elior Malul. About to be drafted into the army, he said he would resist any order to dismantle settlements.
Settler supporters, including many Likud politicians, oppose giving up land they see as a biblical heritage and say Sharon's plan would also "reward Palestinian terror."
Protests over Sharon's pullout
have been ongoing since July
The fear of security forces is that far-rightists will try to use violence to knock the plan, designed to be completed by the end of 2005, off course.
Security has been tightened round Sharon. An assassin killed Prime Minister Yitzhak Rabin in 1995 in an attempt to scupper interim peace accords signed with the Palestinians.
One option for dealing with people suspected of planning violence was "administrative detention", or detention without trial, which has been used against hundreds of Palestinians arrested during a four-year-old uprising, officials said.
"I hope we will not have to use it against settlers or against religious leaders. But we are reserving our right to do so if necessary," Justice Minister Yosef Lapid told reporters.
Sharon's inner cabinet is expected later this week to discuss a law for carrying out the withdrawal plan.