"All options are open in this action. We will take any measure to protect the peace and partnership agreement," signed after the northern militia overran Sanaa in September, Abdul Malik al-Huthi said.
"No one, not the president or anyone else, will be above our measures if they try to implement a conspiracy against this country," he warned in a long televised speech.
Huthi accused Hadi of "protecting corruption" and supporting "the fragmentation" of Yemen, reiterating the Shiites' rejection of a plan to turn the republic into a six-region federation.
His militiamen on Saturday abducted Hadi's chief of staff, Ahmed Awad bin Mubarak, who is overseeing the drafting of a constitution opposed by the militia, mainly over the six-region plan.
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The Shiites, whose stronghold is in the arid northern mountains, have charged that the plan divides the country into rich and poor regions.
The warning came after the rebel fighters stormed the presidential complex in southern Sanaa and clashed with troops protecting the residence of Hadi in western Sanaa.
Huthi also shrugged off any UN Security Council measures against his movement in defence of Hadi's US-backed government.
"I advise the UN Security Council... (that) you will not benefit from any measures you wish to take" against the Shiite rebels, he said.
"We are ready to face the consequences, regardless of what they are," he added, speaking as the Security Council met to discuss the chaos in Yemen.