The man fired a shot and demanded to speak to the elite police unit which shot Islamist gunman Mohamed Merah.
Merah, who claimed to have al-Qaeda training, killed seven people in Toulouse in March before he was shot dead by police.
The director of the bank, a branch of CIC, is said to be among the hostages.
The regional newspaper Ouest-France says the area around the bank has been sealed off by the security forces.
"We do not know if his claim about al-Qaeda is serious or a fantasy," a police union source told Ouest-France.
The BBC's Christian Fraser, in Paris, says the bank is 100 metres (330ft) from Merah's flat.
A police official told the Associated Press no injuries had been reported so far.
Mohamed Merah, a Frenchman of Algerian descent, killed seven people in three separate attacks. His victims included three children and a teacher at a Jewish school, and three soldiers.
He was shot dead by a police sniper on 22 March after commandos stormed his flat.
In the wake of the shootings, the French authorities set up an investigation into whether Merah had accomplices and into possible Islamist indoctrination practices in prisons.
Merah filmed his attacks, footage which was later recovered.
On 7 June, a man armed with a shotgun took hostage a security guard at the French weather service, Meteo France. The hostage-taker fired several shots and was seriously injured when police returned fire.
His hostage was uninjured.
