Undercover Sun investigators secretly recorded Choudary telling a young and impressionable audience that they would eventually rule under strict Sharia law.
Choudary also said it would be easy for vast numbers of Muslims to declare Jihad, or holy war, against Britain - and that every one of them could become "a time bomb waiting to go off".
Choudary, the right-hand man of exiled preacher of hate Omar Bakri, ranted before 100 young cheering Muslims at a meeting in East London.
"About 500 people in Britain become Muslim every day. The Home Office say there are 1.5million Muslims but there were 1.5 million ten years ago. Since then our brothers in Bethnal Green, Whitechapel and other places have had eight or nine children each. Eight children here, ten children, 15 children. There must be at least six million people. It may be by pure conversion that Britain will become an Islamic state. We may never need to conquer it from the outside," he said.
The rally, which was advertised as a debate on whether the West had "learned the lessons" of 9/11, was held amid strict security in a hall above a mosque in Lea Bridge Road, Leyton. Stewards on the door demanded ID from non-Muslims.