Speaking with ISNA, deputy chief of operations for Iran’s Police Force Hossein Sajedi-Nia said the pilgrims have entered Iraq during the past two weeks.
About three million Iranians have registered for the rally on Iran’s SAMAH automation system, a legal process which does away with the need to get an Iraqi visa, Iran’s Central Arbaeeen Committee said, Press TV reported.
Iranian authorities have predicted that more than three million mourners will take part in this year's rally, topping numbers in previous years.
Thousands of non-Iranian pilgrims have also entered Iran, joining the trek for Karbala.
The foreign pilgrims come mostly from Iran’s neighboring countries, such as Pakistan, Afghanistan, Azerbaijan and Turkey.
Earlier this week, Sistan-Baluchestan Police Chief Brigadier General Mohammad Qanbari said that more than 40,000 pilgrims had entered the country from Pakistan alone.
Millions of pilgrims from across the world gather in Iraq’s holy city of Karbala to mark the 40th day after the martyrdom anniversary of Imam Hussein (PBUH), the grandson of the Prophet Muhammad (PBUH).
The gathering is known to attract about 20 million participants every year. This year’s Arbaeen falls on October 19.
This year’s Arbaeen pilgrimage takes place as Iraq is recovering from several days of protests against mismanagement and corruption last week.
Some of the demonstrations descended into violence and bloody clashes. Violent protests have receded in the past days, however.
Last week, some Arab media outlets reported a US-backed plan seeking to influence the protests in a bid to create instability and push certain political agendas in the country.
Observers have pointed out that the timing of the provocations signals foreign-backed elements seek to undermine Arbaeen processions, specifically through fueling antagonism toward Iran.
On Sunday, Leader of the Islamic Revolution Ayatollah Seyyed Ali Khamenei tweeted that "Iran and Iraq are two nations whose hearts & souls are tied together through faith in God”.
The Leader said that an enemy plot seeking to “sow discord” between the two nations has failed and will fail.