ZURICH (Reuters) - Around 100 police officers staked out a suspected armed robber at a Zurich bank for nearly seven hours Monday, only to find the man had already fled minutes after the alarm was raised, police said.

Police stormed the branch of Credit Suisse, opposite a police station, having tried for hours to establish contact with the assailant who was suspected of holding two employees hostage.

The drama was broadcast live on Swiss television, but it seemed the gunman had already given the slip.

"On the basis of current police information, it must be assumed that the suspect fled before the first police officers arrived on the scene," police said in a statement.

Local residents were told to remain indoors and a section of the suburb was cordoned off as police sought to bargain with the man after a third employee raised the alarm.

In fact, the employees, a male trainee and woman, had locked themselves in rooms the gunman could not enter and did not know he had already fled. They were found uninjured but traumatized by the stake-out.