"A awful disaster of people who came to celebrate Lag B'Omer and were unfortunately crushed to death", said United Hatzalah vice president of operations Dov Maisel.
But officials later said it appeared the casualties had been asphyxiated or trampled in a stampede.
"A serious disaster on Mount Meron", said Israeli Prime Minister Benjamin Netanyahu.
Magen David Adom, the Israeli rescue service, tweeted that medics are on the scene treating several injured.
Authorities had authorized 10,000 people to gather at the site of the tomb but organisers said more than 650 buses had been chartered from across the country, bringing 30,000 pilgrims to Meron. "It was a disaster", a witnesses told Haaretz.
At least 44 people have died, many of them crushed to death, at a stampede that broke out at an overcrowded Lag B'Omer event in the north of Israel overnight.
The disaster occurred at Mount Meron at the main celebrations of Lag BaOmer, a holiday when tens of thousands of people, mostly ultra-Orthodox Jews, gather to honour Rabbi Shimon Bar Yochai, a 2nd century sage and mystic who is buried there. Police shut down the site and ordered revellers out.
Scenes from Meron hours after the accident showed an ultra-Orthodox Jewish crowd in distress, the men in long black coats and wearing black hats, and debris scattered across the ground.
European Union sues AstraZeneca after delays in vaccine deliveries
The EU has nearly 448 million residents - more than one hundred million more than the USA population of 328 million. AstraZeneca has so far delivered just 31 million of the 120 million doses it had promised to the EU.
Photos showed rows of wrapped bodies lying on the ground afterward, with dozens of ambulances at the site.
United Hatzalah emergency medical service said that dozens of resuscitations were carried out at the site by the organization's volunteers.
"I have not seen anything like this since I entered into the field of emergency medicine", he added.
"Out of the five that came in helicopters, we have four injured seriously, one is medium", a hospital spokeswoman told The National. The event attracted tens of thousands from the ultra-Orthodox community.
Around 5,000 police were deployed to secure the event, with the police urging pilgrims to avoid incidents during the feast when bonfires are lit.
The area has since been closed, with rescue workers and security forces attempting to clear the area.