Praise be to Allaah.
It is not permissible for a woman to lead men in prayer. The Prophet (peace and blessings of Allaah be upon him) said: “Keep women behind [in the back rows] as Allaah has commanded.” (Narrated by ‘Abd al-Razzaaq in his Musannaf, 5115. The isnaad stops at Ibn Mas’ood with a longer report than this; the isnaad is saheeh but it is not proven that this was said by the Prophet (peace and blessings of Allaah be upon him)).
Moreover, the position of imaam in the mosque is a kind of wilaayah [public office], and wilaayah is only for men. “No nation prospers that appoints a woman over its affairs [wilaayah]” (narrated by al-Bukhaari, 13/45, 46), as the Prophet (peace and blessings of Allaah be upon him) said.
There is an exception made by the Hanbalis, but this is a weak opinion. This view says that a woman may lead the men in taraaweeh if she reads well and the men present are illiterate, but she should be behind them and they should be in front of her. But there is no evidence (daleel) to support this view. The point is that it is not permissible for a woman to lead men in prayer. Yes, women may lead other women in prayer, it is OK if she leads other women. This is fine, as stated in the report of Umm Waraqah leading some of her mahrams, but as for her leading non-mahram men or as a public office [wilaayah], such as being an imaam in a mosque, this is not permitted.