TABRIZ, Iran  The strongest challenger to President Mahmoud Ahmadinejad attracted an unusually large and exuberant crowd of supporters on Monday during a campaign speech in this northwest city near the candidate’s birthplace, with only a few weeks before national elections that the incumbent stands a serious chance of losing.

The crowd for the challenger, Mir Hussein Moussavi, was extraordinary not only for its size  an estimated 30,000  but also because the supporters were not paid, given free food, bused in or ordered by their workplaces to attend, a tactic sometimes used by Mr. Ahmadinejad’s campaign.

Many traveled here in private cars and learned about the rally despite new government restrictions on Facebook, the social networking site, which Mr. Moussavi’s campaign had been using to spread word of his candidacy among the country’s predominantly young electorate. The supporters gave a rousing welcome to Mr. Moussavi, who was born in Khameneh, a small town in the Azerbaijan area of Iran.

“Azerbaijan is my home; Moussavi is my life!” they chanted to him in their native dialect, Turkish Azeri.