RAMALLAH, West Bank ? Jailed Palestinian uprising leader Marwan Barghouti declared his candidacy for president Wednesday, a stunning last-minute reversal that shook up Palestinian politics ahead of the Jan. 9 vote for Yasser Arafat’s replacement.

Adding to the uncertainty, the militant group Hamas said it would boycott the election. It was the first sign of open divisions between the interim Palestinian leadership and the Islamic opposition group since Arafat’s death Nov. 11.

The moves injected drama into what has been a smooth transition of power. Before Wednesday, interim leader Mahmoud Abbas managed to win pledges of unity from the disparate Palestinian factions and seemed a shoo-in to win.

The fiery, charismatic Barghouti, who is serving five life terms in an Israeli prison for his role in deadly attacks, is far more popular among young Palestinians than the staid Abbas.

Last week, Barghouti sent a message from his prison cell saying that he would not pursue the presidency for the sake of unity in the ruling Fatah movement. But Wednesday, he abruptly changed his mind.

Abbas already has been nominated as Fatah’s presidential candidate, so Barghouti must run as an independent. But as a leading Fatah member, he would likely undermine Abbas’ prospects.

When the Palestinian uprising broke out in September 2000, Barghouti used Arab satellite television to turn himself into the most prominent voice of the Palestinian resistance. Though he said he still supported a peaceful solution, Barghouti said force — including shooting attacks on Israelis — was justified to end Israel’s occupation of the West Bank and Gaza Strip.

Israeli arrested Barghouti in 2002, accusing him of funneling money to militants and being involved in attacks that killed 26 Israelis. He was convicted of attacks leading to five deaths.

Abbas, who formally launched his campaign Wednesday in Ramallah, brushed off Hamas’ decision to boycott the election and signaled that he was ready to take action against the militants.

“Every nation has opposition groups, but there are also laws and institutions,” he said. “I am committed to having one authority and only one army.”