What happens in Jerusalem at the start of the tribulation?
At the start of the tribulation, God's two witnesses appear in Jerusalem and preach to the nation of Israel. Their preaching leads to the setting aside and sealing of 144,000 Jews from all the tribes of Israel, shortly after the seven years of tribulation begin.
Les explains that as soon as the tribulation has begun, one of the first things that happens in Jerusalem is the appearance of God's two witnesses, who are going to preach to the nation of Israel. Out of their preaching, the first thing that happens is the setting aside of the 144,000 Jews. He emphasizes that these 144,000 are Jews, sealed from all the tribes of the children of Israel, not just two tribes, and this occurs shortly after the opening of the seven years of tribulation. Even though Israel has been dispersed among the nations for almost 2,700 years, they are still Jews, and God knows what tribe they came from even when they themselves cannot tell.