At least 26 people were killed when a mosque and a police station came under attack in the Iraqi capital Baghdad.
The mosque was attacked as worshippers finished dawn prayers
A car bomb exploded outside a Shia mosque in a Sunni district, killing at least 14 worshippers and wounding 19.
A short while earlier, rebels fired mortars and then stormed a police station, killing at least 12 people - mainly police officers.
Friday's violence - coming as the city began the Islamic weekend - ends a period of relative calm in Baghdad.
An Islamic website carried a statement claiming responsibility for the attacks, purportedly from al-Qaeda linked militant Abu Musab al-Zarqawi.
In other developments:
- The US military announces an extra 12,000 troops will begin heading to Iraq on Friday, to boost security ahead of January's planned elections
- Three Iraqis are arrested in Germany on suspicion of planning an attack on Iraqi Prime Minister Iyad Allawi, who is in the country for talks
- At least 11 insurgents and a policeman die in clashes between militants and US/Iraqi forces in the northern city of Mosul, the Associated Press reports
- A US soldier is killed in an attack on a convoy near another northern city, Kirkuk
- A second US soldier dies in an explosion in Baghdad, the US military say
- Nato Secretary-General Jaap de Hoop Scheffer visits the alliance's military training mission in Iraq
- Iraq interim President Ghazi al-Yawar is on his way to the US for an official visit, and is due to meet President George W Bush
Amid political turmoil in Ukraine, the parliament demands the withdrawal of its 1,600 troops from Iraq
The blast in the Sunni Muslim area of Adhamiya happened at about 0700 (0400 GMT), an interior ministry spokesman told the AFP news agency.
He said a suicide bomber detonated explosives in a car near the Hamid al-Alwan mosque as people were leaving the building after morning prayers.
Witness Mahmud Fuad, who lost a brother in the attack, said there were two explosions.
"First there was an explosion in a car. Two other cars caught
fire and people rushed to put the blaze out. That was when a second explosion killed many residents," he was quoted by AFP news agency as saying.
'Attacked from all sides'
Rebels reportedly fired mortar rounds into the police station in al-Ummal district before storming the courtyard at about 0600 local time (0300 GMT).
A resident, Ahmed Hashem, said armed men approached the police station "from all sides".
"I saw armed men firing towards the police station while taking cover behind rubbish bins and the police ran away," he told AFP.
Almost all the policemen on duty at the time were killed, the armoury was looted and more than 50 prisoners were set free, the BBC was told.
The BBC's Peter Greste in Baghdad says attacks on police stations are not uncommon, but Friday's was unusual because of its audacity.
The police station is close to the main airport highway - one of the most dangerous stretches of road in the country. It is heavily fortified and its police officers well armed.
Similar tactics were used in Mosul last month, our correspondent adds. There, rebels overran at least nine police stations, attacking security posts, stealing weapons, flak jackets and police vehicles.
Iraqi police and security forces are frequent targets of insurgents fighting US-led forces and the interim government.
The BBC's Arab affairs analyst, Magdi Abdelhadi, says Friday's violence in Baghdad is another sign of insurgents' aim to make it very difficult to stabilise the country.
By attacking the police station, he says, they are hoping to intimidate and deter those who want to join the Iraqi police force - a key part of America's exit strategy from the country.
He adds that the attack on the Shia mosque appears to be an attempt to ignite sectarian strife, which could ultimately derail the election.