They started making their way there in any way they could.
Many walked. In their distinctive, signature black coats and hats they took the most direct route, even if that meant striding along on the hard shoulder of the busy city roads.
Others began their journey early in the morning, heading to Jerusalem from towns and cities all over Israel.
By four o'clock, there were hundreds of thousands on the streets of West Jerusalem, stretching back through the haze of the late afternoon for as far as the eye could see.
A unique meeting of the three mainstream ultra-Orthodox Jewish movements called the demonstration just a few days ago, but so confident of their support, they boldly predicted a turnout of half a million.
There seemed to be that, maybe more. It was hard to judge, but this is a community united by anger at government plans which challenge their way of life by demanding they serve in the military.
Since the foundation of the state, the ultra-Orthodox, known as the Haredi, have been exempt from military conscription. Having seen the widespread destruction of Jewish schools across Europe during World War II and the Holocaust, the new country's leaders agreed to let a few hundred men rebuild the idea of scholarship and biblical study.
From a few hundred given a pass, it's grown over the years to hundreds of thousands all paid by the government to study the Torah and pray for Israel. The mass exemption was meant to end two years ago, but it didn't. Politicians were reluctant to stand against such a vocal and powerful lobby. The Haredi make up around eight percent of Israel's eight million citizens.
But there has been a political shift in the country in the past year. The ultra-Orthodox political parties found themselves no longer welcome in government, replaced by a centre-left coalition.
And it's demanding changes, the biggest being ultra-Orthodox men do what every other Israeli does, and serve in the military.
The Haredi believes that could put the very spirit of Israel at risk.
"We contribute to society through our actions, our studies and prayer and we believe that the state has survived through our efforts," I was told by Gideon Katz, an appointed spokesman for the gathering.
I decide to walk through the crowd, but it's hard to get people to speak. There is a suspicion of the media a fear their views are distorted and misunderstood in a society suspicious or wary of such devotion.
One young man tells me he will leave the country rather than be forced to leave his studies.
"I feel they are forcing me to choose my country or my religion and I would choose my religion," he said.
I ask where he would go. "America. I would ask for asylum. They respect the freedom of religion there. I'm told some senators support the idea."
Another man approaches me and thanks me for the interest in the demonstration.
"I think we should try to understand all sides of this argument, but in a country that is built on its religion, it is important we allow people to study the Torah, to pray. We don't want to lose our spiritual identity," he said.
Not all Haredi are opposed to enlistment, and over the past two years, the majority of men who've received call-up papers have gone to a processing office, and so avoided arrest for draft dodging.
The government understands the opposition to its plans and hopes it can find a compromise. But the ultra-Orthodox are not about to back down and insist they will continue to show their strength on the streets of the country they insist they protect.