What was Ezra doing when a very large assembly of men, women and children gathered around him, weeping?

Answer: Praying and confessing

As Ezra prayed and confessed, weeping and falling facedown before the house of God, a large crowd gathered and wept bitterly too.

“While Ezra prayed and made this confession, weeping and falling facedown before the house of God, a very large assembly of Israelites—men, women, and children—gathered around him, and the people wept bitterly as well.”— Ezra 10:1 (BSB)