CAIRO — There are few anecdotes about him, and pictures, at least ones that have appeared in public, are scarce. But Mojtaba Khamenei, the son of Iran’s supreme leader, wields considerable power and is a key figure in orchestrating the crackdown against anti-government protesters, analysts say. The younger Khamenei operates tucked behind an elaborate security structure, an overlapping world that stretches from Iran’s Revolutionary Guard corps to the motorcycle-riding Basiji militiamen. Analysts and former dissidents describe him as the gatekeeper for his father, Ayatollah Ali Khamenei, a reclusive son whose political instincts were sharpened in a post-revolutionary Iran where affiliations with security and intelligence services were just as important as Islamic ideology. The anxiety in the streets of Tehran today goes deeper than the outrage over the June 12 election that authorities say was won by President Mahmoud Ahmadinejad — it is the newest round in a struggle between hard-liners and reformists that began more than 20 years ago over the legacy of the 1979 Islamic Revolution. And at the center, or at least very close to …