The bodies of 23 men were found dumped at six sites, most of them in predominantly Shia parts of Baghdad, police said on Thursday.
The victims were all shot and most had their hands bound, police said.
The grisly finds come amid reprisal attacks for the bombing on Wednesday of a revered Shia shrine in Samarra. It was not immediately clear whether the killings were linked to that attack.
The bodies of three Iraqi journalists, including a well-known correspondent for Al-Arabiya television, were found on Thursday near Samarra, police and the Arabic network said.
Al Arabiya's Atwar Bahjat and two colleagues from another media company were in the city to cover the bombing of the Shia shrine. Their employers lost contact with them on Wednesday night.
police Captain Laith Muhammad said their bodies were found near their vehicle, cameras and satellite dish. They had been shot.
In another incident, armed men in police uniforms seized 11 Sunni men from a prison in the mainly Shia city of Basra on Wednesday and later killed them, police said.
Among those killed in the apparent reprisal attack were two Egyptians. The others were Iraqis.
The damaged shrine contains the
tombs of two Shia imams
Earlier on Wednesday, unidentified attackers blew up the golden dome of the Askariya shrine in Samarra. The bombing triggered more than 90 reprisal attacks on Sunni mosques.
In Washington, a senior US official said the al-Qaida was suspected of being behind the attack on the Shia shrine.
"We believe this can be traced back to the Zarqawi al-Qaida movement," said the State Department's coordinator for Iraq policy, Ambassador James Jeffrey, on Wednesday adding the United States would do all it could to track down the perpetrators.
Asked what evidence the US had to link the attack to al-Qaida, Jeffrey said Abu Musab al-Zarqawi, the al-Qaida leader in Iraq, had often called for attacks on Shia targets and said they were aimed at sparking civil war.
"We are trying to connect the dots," he told reporters at the State Department. "We certainly think it would be in line with what they have been saying and doing."
Earlier on Wednesday, the president said extremists were pushing the country towards civil war, as many Shia lashed out at the United States.
As the dome of the 1200-year-old Askariya shrine lay in ruins, leaders on both sides called for calm.
"We are facing a major conspiracy that is targeting Iraq's unity," said Jalal Talabani, the president and a Kurd. "We should all stand hand in hand to prevent the danger of a civil war."
Zalmay Khalilzad, the US ambassador, and the top American commander in Iraq, said it was a "critical moment for Iraq" and called the bombings a deliberate attempt to create sectarian tension. They promised the US would contribute to the shrine's reconstruction.
"This attack is a crime against humanity," Khalilzad and General George Casey said in a joint statement.
Iraq's top Shia cleric and the country's vice-president hinted that local militias might play a bigger role in security in the future if the government cannot protect holy shrines.
Some Shia political leaders already were angry with the United States because it has urged them to form a unity government in which nonsectarian figures control the army and police. Khalilzad said earlier this week that the US would not continue to support institutions run by sectarian groups with links to militias.
"We are facing a major conspiracy that is targeting Iraq's unity"
After the attacks, one top Shia political leader accused Khalilzad of sharing some responsibility for the bombing of the shrine because of that stance.
"These statements ... gave green lights to terrorist groups. And, therefore, he shares in part of the responsibility," said Abdul-Aziz al-Hakim, head of the Supreme Council for the Islamic Revolution in Iraq and the former commander of its militia.
The Askariya shrine, also known as the Golden Mosque, contains the tombs of two revered Shia imams, who are considered by Shia to be among the successors of the Prophet Muhammad.
No group claimed responsibility for the 6.55am assault on the shrine in Samarra, 96km north of Baghdad, but a government statement said several suspects had been detained.
Shia protesters have
condemned the attack
In the hours after the bombing, more than 90 Sunni mosques were attacked with automatic rifles and rocket-propelled grenades, burned or taken over by Shia, said the Iraqi Islamic Party, the country's largest Sunni political group.
At least seven people, including three clerics, were killed in the attacks, which were mainly in Baghdad and predominantly Shia provinces south of the country, the party said.
Major Sunni groups joined in condemning the attack. The Association of Muslim Scholars called the bombing a criminal act, while the Sunni Endowment, a government organisation that cares for Sunni mosques and shrines, sent a delegation to Samarra to investigate.
Al-Sistani, the leading Shia cleric, sent instructions to his followers forbidding attacks on Sunni mosques. He called for seven days of mourning, his aides said. But he later hinted, as did Vice President Adil Abdul-Mahdi, that religious militias could be given a bigger security role.
Large protests erupted in Shia parts of Baghdad and in cities throughout the Shia heartland to the south. In Basra, Shia community members traded rifle and rocket-propelled grenade fire with guards at the office of the Iraqi Islamic Party.
A Sunni Endowments group will
travel to Samarra to investigate
Shia protesters later set fire to a Sunni shrine containing the seventh century tomb of Talha bin Obeid-Allah, a companion of Prophet Muhammad, on the outskirts of the southern city. Police found 11 bodies of Sunni Muslims, most of them shot in the head, in Basra, police Captain Mushtaq Kadhim said.
The Interior Ministry said the Samarra attack was carried out by four men, one wearing a military uniform and three clad in black, who entered the mosque and detonated two bombs. The top of the dome collapsed into a crumbly mess, leaving just traces of gold showing through the rubble.
Part of the shrine's tiled northern wall also was damaged.