New Iraqi Prime Minister Iyad Allawi has vowed to crush insurgents after one of the most violent days for months.
Insecurity still plagues Iraq in the run-up to the handover
Days before the handover of power, Mr Allawi said he expected more attacks but insisted the rebels would be confronted and defeated.
US Secretary of State Colin Powell told the BBC the US had underestimated the seriousness of the insurgency.
More than 100 people died and hundreds more were wounded in a wave of attacks in five Iraqi cities on Thursday.
In continuing outbreaks of violence, three policemen died in an attack on a police station in Baquba overnight; US troops killed two militants in clashes in Falluja and a policeman was killed by a roadside bomb in Baghdad.
Mr Allawi said he believed Thursday's attacks were carried out by a group linked to Abu Musab al-Zarqawi, whom the US has accused of leading al-Qaeda militants inside Iraq.
Zarqawi's followers claimed responsibility for the attacks in a statement on an Islamic website.
Braced for escalation
Mr Allawi said the attacks were meant to sabotage the transfer of power from the US-led coalition to an interim Iraqi government on 30 June.
"These are isolated incidents. We are going to defeat them... We have been expecting this escalation and we are expecting more escalation in the days ahead."
MONTH OF VIOLENCE
24 June: At least 100 die in rebel attacks in five Iraqi cities
22 June: US says it kills 20 militants in Falluja air strike
21 June: Four US soldiers die in Ramadi ambush
19 June: At least 20 die in US raid targeting militants in Falluja
17 June: 41 die in car bomb attacks in Baghdad
16 June: Iraqi oil fields security chief killed in Kirkuk
14 June: 12 die in Baghdad car bomb
13 June: Education ministry official killed in Baghdad
12 June: Interim deputy foreign minister Bassam Qubba killed in Baghdad
8 June: Car bombs kill 15 in Mosul and Baquba
By the handover of formal sovereignty, the UN-approved interim government under Prime Minister Allawi and President Ghazi Yawer will have already been in place for a month.
The BBC's David Bamford says in that time Mr Allawi has not been shy in setting out where he thinks the process should go, and has signalled some key changes of direction from the policies of US administrator Paul Bremer.
Some ministries, he adds, have been up and running under Iraqi control for months and the coalition played up each handover as a transfer of sovereignty.
In a BBC interview, Mr Powell said those behind Thursday's attacks were trying to torpedo the handover.
"I think we underestimated the nature of the insurgency that we might face during this period," he said.
"The insurgency that we're looking at now has become a serious problem for us, but it's a problem that we will deal with."
But he said he hoped violence would tail off after the handover, once Iraqis saw that they had their own people in charge.
'Agents and spies'
The worst of the attacks came in the city of Mosul, where at least 62 people died and 220 were hurt in a series of car bombings, the US military said.
More than 40 people died in Baquba, Ramadi, Falluja and Baghdad, in what appeared to be co-ordinated attacks.
Mosul: 62 dead and 220 hurt
Ramadi and Falluja: 20 dead and 76 wounded
Baquba: 13 dead and 15 wounded
Baghdad: 8 dead and 13 wounded
Sources: US military and Iraqi health ministry
Three US soldiers were killed in the raids in the Sunni Muslim heartland, which began at dawn when insurgents seized police stations in Baquba.
In a statement on an Islamic website, Zarqawi's followers said: "Your brothers in Jamaat al-Tawhid and Jihad launched a wide assault in several governorates in the country which included strikes against the apostate police agents and spies
and the Iraq army, alongside their American brothers."
The US has accused Jordanian-born Zarqawi of leading al-Qaeda operations in Iraq and has stepped up its hunt for him in recent weeks.
Will the new authority be legitimate?
The BBC's Middle East analyst, Roger Hardy, says it is not clear that there is a single command centre organising the violence and that Zarqawi's group may be one among many.