BEIRUT — Iran launched on Sunday what it described as its biggest air defense drill ever with the aim of preparing to protect the country’s nuclear sites from possible airstrikes as international talks to resolve the long stalemate over the nation’s atomic research program falter.

Meanwhile, domestic opposition leader Mir Hossein Mousavi continued to put pressure on the government of President Mahmoud Ahmadinejad, praising protesters for turning an annual march against the U.S. into an anti-government rally that met with a violent response by security forces.

“What we saw in the streets was a huge campaign against the people,” he said in an interview posted to his Web site. “Throughout the history of the revolution, I had never seen such a scene and such deployment of so many forces. (It) showed how they fear this movement and what grandeur it possesses.”

Mousavi has been the figurehead of an opposition movement that sprang out of Iran’s disputed June 12 election, in which he was declared the runner-up. Analysts say the ongoing crisis of domestic political authority has complicated international efforts to forge a diplomatic compromise with Iran over its nuclear program, which the West suspects is aimed at eventually producing weapons.

The tussle over what Tehran insists is a civilian nuclear program has become wedded to Iran’s domestic factional battles. Both hard-liners and moderates have criticized an international proposal to exchange the bulk of Iran’s potentially dual-use nuclear material for fuel rods fitted for a Tehran medical reactor, although Ahmadinejad and his closest allies have praised it.