On Monday, tribal leader Naim al-Kuoud al-Nimrawi told AFP that IS "executed 36 people, including four women and three children" on Sunday alone.
Accounts have varied as to the number and timings of the executions, but sources have spoken of more than 200 people murdered in recent days.
A police officer and an official gave figures of more than 200 to 258 people killed, while Iraq's human rights ministry put the toll at 322 and a tribal leader said 381 were executed.
The mass killings appear aimed at discouraging resistance from powerful tribes in Anbar, where IS overran large areas in June as pro-government forces suffered a string of setbacks.
Hundreds of thousands of pilgrims are expected in Karbala for the Tuesday peak of Ashura marking the death of Imam Hussein, one of Shiite Islam's most revered figures.
A roadside bomb in Nahrawan, outside Baghdad, killed at least three people and wounded at least 10 near a tent where Shiites were serving refreshments on the occasion of Ashura.
It came a day after at least 19 people were killed in blasts targeting Shiites in Baghdad.
Karbala deputy governor Jassem al-Fatlawi said "hundreds of thousands of Iraqi pilgrims" and 65,000 others from 20 different countries have thronged the shrine city.
- IS calls Shiites 'heretics' -
Pilgrims have been targeted during Ashura before, but this year they face even greater danger after the IS lightning offensive in June.
Like other Sunni extremists, IS considers Shiites heretics.
Authorities have deployed thousands of security personnel and allied militiamen to protect the pilgrims, in a major test for the new government headed by Prime Minister Haidar al-Abadi.
"The security plan is fully in effect and the security forces are on a state of high alert," a police colonel said.
Police were deployed throughout Shiite districts of Baghdad and security forces are guarding the 100-kilometre (60-mile) route from the capital to Karbala.
More than 26,000 members of the security forces were deployed in Karbala itself, backed by helicopters, army Staff Lieutenant General Othman al-Ghanimi said.
Police used X-ray trucks to scan vehicles as sniffer dogs monitored arrivals and some 1,500 policewomen checked female pilgrims.
The Sunni extremist IS group has declared a "caliphate" in parts of Iraq and neighbouring Syria under its control, imposing its harsh interpretation of Islamic law and committing widespread atrocities.
There are fears that Anbar province, stretching from Iraq's borders with Jordan and Saudi Arabia to the western approach to Baghdad, could fall entirely.
- Clashes, strikes in Kobane -
A US-led coalition of Western and Arab nations has carried out a wave of air strikes on IS positions in Iraq and Syria, with Canadian CF-18s conducting their first raids on Sunday around the Iraqi city of Fallujah.
The Pentagon said its aircraft carried out five strikes on Sunday and Monday around Syria and nine in Iraq.
Kurdish militia have been holding off an IS offensive on the Syrian town of Kobane for nearly seven weeks and have been reinforced by about 150 heavily-armed Iraqi Kurdish peshmerga fighters.
Kobane, just across the border from Turkey, has become both a key symbol of resistance to the IS advance and a rallying point for the jihadist cause.
At the weekend, the peshmerga targeted IS positions with rockets, according to Syrian Kurdish militia defending the town.
The Syrian Observatory for Human Rights said 20 jihadists were killed in coalition strikes in Kobane and elsewhere, and that at least four IS militants and two Kurdish fighters died Sunday.
The Britain-based monitor also said IS beheaded eight Syrian rebels it captured in the eastern province of Deir Ezzor and hung their bodies on makeshift crucifixes.