On the road to Karbala with Iran's young pilgrims
Let that sink in. 22 million people is only just short of the population of Australia.
That's 10 times more people than completed the Hajj this year. It constitutes the biggest annual gathering on earth.
The event is for the Shia observation of Arba'een, the last day of an annual 40-day commemoration of a 680AD battlefield obliteration in Karbala, in which Hussain, the heavily outnumbered third Imam of Shiism, came to a particularly gruesome end at the hands of a despised and brutal tyrant named Yazid.
Even after 1,300 years, the enduring story of Hussain's fight against the odds is still so raw that it constitutes an emotive force keeping disparate and minority Shia communities mobilised and fiercely protective of their collective identity.
The Baathist dictator Saddam Hussain banned the Arba'een pilgrimage - and in the years since his fall, it has evolved from a local statement of religious defiance into the international phenomenon we see now.
Iran has a heavy presence at the event - around two million Iranians took part in 2016 - leading some of Iran's less religiously inclined to worry that the event's peaceful message could be overshadowed by the accusation of Iran's involvement constituting a provocative statement of regional political ambition.
Those walking do so in spite of the considerable risk of suicide bombs.
For the Islamic State group, the Shia are "rafidi" heretics and thus "legitimate" targets for violence. Last year, a suicide bomb killed more than 80 pilgrims. It's a testament to the formidable security arrangements put in place by the Iraqi government that the number of successful attacks is not substantially greater.
Last November, while studying for my masters in Tehran, I decided to join the pilgrimage to get an insight into the extent of Iran's presence. As a conspicuously tall, blue-eyed non-Muslim Brit, I was keen to be as invisible as possible, and persuaded a group of 270 young Tehrani teachers to let me tag along with them.
For six days I wore the chador (Iran's iconic long black cloak) and immersed myself in their socially conservative, deeply patriarchal and religiously devout bubble.
The scale of the pilgrimage was epic. Arriving at Najaf airport, the masses of Pakistani, Lebanese, Azerbaijani and even Saudi Arabian Shia spilling out of the visa queues was a sign of the international fluidity that was to come.
The 82km road to Karbala was a thick moving block of black chadors, bearded men and flags: flags of the many nationalities taking part and a colourful array of religious flags reading "Ya Hussain". The women all wore the black chador or niqab. As a sign of respect to the sombre nature of Arba'een, none wore colour.
It was difficult to discern the individuals making up the collective, or to determine who was rich, who was urban, who was rural, who was educated, who was illiterate. Most people walked in cheap plastic sandals and a huge number walked in bare feet.
The route was lined with giant tents and locals provided a never-ending supply of tea, water and hot food to the 22 million visitors. Pilgrims of all nationalities walked side-by-side in a massive sprawl of humanity, absorbed in prayer and grief for their hero.
They cried, beat their chests, hit their foreheads and struck the backs of their shoulder blades with blunt chains. There were barefoot Pakistani ladies, deep in prayer, power-walking and clutching speakers blaring out their favourite mourning singers.
As intense and alien as this scene was to an outsider, the pilgrimage was a family event. There were toddlers, pushchairs, wheelchairs and war veterans hobbling along the road on crutches. There were massage and foot-rub stations set up along the route and the Red Crescent were always on hand for medical assistance.
Moreover, despite the frenzy of emotions being channelled in the mourning rituals, to my relief the demonstrations were entirely peaceful without a hint of aggression or anger.
After five long days of walking and five short, sleepless nights sharing tents with up to 300 women and crying babies, by the time we arrived in Karbala I was ruined: emotionally and physically exhausted, sneezing and sporting impressive dreadlocks in my seven-day unwashed hijab hair.
The climax of our journey was our visit to Hussain's shrine, a huge rectangle encrusted with mirrors and rammed to bursting point with pilgrims. My group made its way through collectively in a long noisy line, banging their drums, beating their chests and crying hysterically.
So what did it all mean?
According to the women with whom I spent every waking moment of the 82km journey, "we make this journey for Hussain... the journey teaches us to be better Muslims, better people".
Many of them had been many times before and the event was clearly the highlight of their calendar. For these young Iranians it was a spiritual and sublime journey, but also a rare opportunity to get a visa to a foreign country, a chance to travel with friends - without their parents - and congregate with people from all over the world in the name of their faith.
And the significance of Iran's heavy presence at the pilgrimage? It's true that there was ubiquitous imagery along the route celebrating the Islamic Republic's heroes - especially Ghasem Soleimani, Ayatollah Khomeini and Ayatollah Khamenei - and imagery denouncing the tyranny of Iran's sworn enemies: the Islamic State group, the United States and Israel.
The not-so-subtle message of the flags, signs and songs: the Shia are fighting for justice against a modern day tyranny, against the odds, just like the hero Hussain in 680AD. My group leader clarified this "modern tyranny" using the analogy of a tree: Wahhabism and IS as branches, with the US government at the root.
From the point of view of this observer, the biggest statement of the pilgrimage was its internationalism, which transcended any accusations of Iranian manipulation.
Yes, Iran had a heavy presence. But it was dwarfed by Iraq's presence. Furthermore, the collective presence of Pakistani, Afghan, Azerbaijani, Indian and Arab pilgrims gave the pilgrimage such a multiethnic, multicultural character that any potential Iranian political agenda was overwhelmingly obscured by the diversity of the pilgrims and the unifying religious significance of the event.
There was certainly no flag burning, and those Pakistani ladies with their speakers were not marching to Iran's tune.
Charlotte Phillips has lived in Tehran for the past two years and recently completed a masters degree at the University of Tehran, majoring in Iran's water problems. Before moving there she worked as project finance lawyer at Linklaters LLP, London.