The governing Palestinian movement Hamas has said it will boycott early elections called by Palestinian Authority President Mahmoud Abbas.
Khaled Meshaal said Hamas would oppose the elections peacefully
Hamas political leader Khaled Meshaal said Mr Abbas's action was illegal.
Tony Blair, who is in the region, has backed the poll call. Israeli Prime Minister Ehud Olmert said "everything should be done" to support Mr Abbas.
A member of Mr Abbas's Fatah movement has been shot dead, putting pressure on a day-old truce with its rival Hamas.
Fatah said the man had been killed during an attempt to free three of its members kidnapped on Monday. Three others were hurt in the gunfight in Jabaliya refugee camp, northern Gaza.
WEEK OF TENSIONS
9 Dec - Mr Abbas' suggestion of early polls is backed by PLO leaders; Hamas denounces the idea
10 Dec - Gunmen fire on the convoy of the Hamas interior minister
11 Dec - Three sons of a Fatah security chief are shot on their way to school; both Fatah and Hamas condemn the killings
14 Dec - Hamas PM Ismail Haniya's convoy comes under fire as he returns from Egypt, killing a bodyguard
15 Dec - Hamas accuses key Fatah figure Mohammad Dahlan of being behind the attack on Mr Haniya, a charge he rejects
16 Dec - Mr Abbas announces he will call early elections; Hamas calls the move a "coup"
17 Dec - A truce is called following street battles between Hamas and Fatah supporters in Gaza, in which three people die
Meanwhile, Hamas's Prime Minister Ismail Haniya called on Mr Abbas to withdraw his security forces from Gaza's streets, saying their deployment endangered the truce, Reuters news agency reports.
Efforts are continuing behind the scenes to firm up the fragile verbal truce agreement, negotiated by mediators on Sunday without a meeting between Hamas and Fatah leaders.
Fighting between the two factions has paralysed the administration.
Mr Abbas's call for early elections on Saturday was labelled a "coup" by Hamas, which won a sweeping victory in parliamentary elections in January and controls the government.
The Palestinians have faced an international aid boycott since because of Hamas' refusal to renounce violence or recognise Israel.
Mr Abbas said he had called for early presidential and legislative elections to relieve "the current impasse".
"I felt it was essential to allow the people to have their say on a platform that achieves Palestinian national interests," Mr Abbas said.
But Mr Meshaal - who is based in Damascus - told the BBC that the president's actions were illegal.
"We stand against any step that is against the law and against the constitution," Mr Meshaal said.
"What's needed to address the inter-Palestinian crisis is national agreement, not actions by individuals, especially taken in response to foreign pressure."
There have been a series of clashes between members of Hamas and Fatah in recent days. On Sunday a ceasefire was reported, but outbreaks of violence continued through the night and into Monday.
Mr Meshaal said Hamas would take practical steps to stop early elections taking place, using "peaceful, popular pressure - not with violence".
In talks in Ramallah on Monday, Mr Abbas urged Mr Blair, the British prime minister, to lift an aid freeze on the Palestinians and said he was ready to hold "serious" talks with Israeli PM Ehud Olmert.
Mr Blair praised Mr Abbas and urged the international community to back a new push towards peace in the region.
"Your people are suffering," Mr Blair told Abbas. "We don't want anything to stand in the way of helping the Palestinian people."
Mr Blair also told the Palestinian leader he backed his efforts to revive the peace process.
"Nobody should have a veto on progress," the prime minister said.
Tony Blair and Ehud Olmert both backed Mr Abbas' call for elections
The BBC's Wyre Davies in Jerusalem says it appears as if Mr Abbas has called new elections almost out of desperation.
The move is a controversial one, our correspondent adds, but is supported by Mr Blair as, in his view, the only realistic way to achieve a two-state solution in which Israel and the Palestinians work towards mutual recognition.
Speaking at a later news conference with Mr Blair, Prime Minister Olmert said "everything should be done" to support Mr Abbas.
He said he hoped to meet the Palestinian president "very soon".