By Mushtaq Mohammad
KERBALA, Iraq (Reuters) - From across Iraq and neighboring states, millions of Shi'ite pilgrims are heading this week to the city of Kerbala for a religious ceremony that authorities say radical Sunni fighters are targeting for attack.
Already hundreds of thousands of Shi'ite faithful, many from adjacent Iran, have visited Kerbala for rituals which culminate in Saturday's Arbain holy day -- the last of 40 days' mourning for the death in battle of Imam Hussein 13 centuries ago.
Roads and highways across Iraq have been filled with black-clad pilgrims heading on foot to Kerbala, a journey which can take days, carrying banners bearing Hussein's image.
Arbain, a defining ritual of Shi'ite Islam and its rift with Sunnism, has frequently triggered militant attacks. This year it unfolds for the first time since Islamic State Sunni Muslim fighters seized control of much of north and west Iraq.
"We have information that they will try to infiltrate crowds of pilgrims and kill civilians everywhere," Shi'ite Prime Minister Haider al-Abadi said this week during a visit to Kerbala, about 50 miles (80 km) south-west of Baghdad.
Abadi said security forces would thwart any attempt to disrupt Arbain, but they face a double challenge.
Not only are many troops diverted to tackle Islamic State fighters elsewhere -- forcing authorities to rely more heavily on Shi'ite militia to keep order -- but this year's flood of foreign visitors has been swollen by Iraq's decision to ease visa requirements for Arbain pilgrims.
An immigration official said visa fees, which normally cost $40, had been waived entirely and video from the Shalamja border crossing with Iran, 10 miles (15 km) east of Basra, has shown thousands of people pouring across as the gates open every day.
Speaking on Tuesday, a senior border official said 200,000 Iranians had already crossed, initially overwhelming his staff.
"This is the first time in the history of this border crossing that we've had such intensity," said Shalamja frontier director Major General Saddam Abdul Sahib.
He said the last comparable surge came after the 2003 U.S.-led invasion which toppled Sunni dictator Saddam Hussein, who fought an eight-year war with Iran and banned major Arbain commemorations. Around 100,000 crossed at that time.
Kerbala deputy governor Jassim Abid said 2 million foreigners had already reached Kerbala, mainly Iranians, and millions more had come from inside Iraq, where the majority of people are Shi'ites.
Alwiya Najat, an Iranian pilgrim in Kerbala, said she only decided to come when she heard of the "miracle" of the visa fee being waived. "I just gathered my family and got our clothes. We hadn't prepared but we just came," she said.
(Additional reporting by Saif Hameed and Ahmed Rasheed in Baghad; Writing by Dominic Evans; Editing by Crispian Balmer)