Hamas, in its first rift with Fatah since forming a Palestinian unity government, accused President Mahmoud Abbas on Monday of acting illegally by naming one of the Islamist group's foes as national security adviser.
Mohammad Dahlan, one of Fatah's most powerful leaders and a possible successor to Abbas, led a crackdown on Hamas Islamists as Gaza security chief in the 1990s.
In recent months, Hamas accused Dahlan of trying to assassinate Prime Minister Ismail Haniyeh of Hamas and of unofficially commanding pro-Fatah forces in fierce factional fighting before the unity government was sealed.
"Hamas sees the step by President Mahmoud Abbas to appoint Mohammad Dahlan, a lawmaker, as his adviser for national security as a violation of Palestinian law," Hamas said in a statement.
"The president of the authority did not consult Hamas or anybody else when he took the decision," Hamas spokesman Fawzi Barhoum said. "We hope that the president will reconsider this decree."
Appointing Dahlan was Abbas's first official act after swearing in the new government on Saturday, giving him a key role in the chaotic security hierarchy at a time when both sides say they want to end internal warfare.