Police efforts to harness the technology of Twitter to keep the public informed went a little awry during a bank siege in Watford.

The Co-operative bank in Market Street was cordoned off and people were evacuated from nearby buildings after a 23-year-old man threatened staff and refused to leave the branch at around 10am.

As the stand-off with armed police grew protracted, Hertfordshire police refused to comment on rumours that the man was holding an explosive device.

But the force then began posting updates about the siege on Twitter, under the giveaway hashtag #watfordbomb.

Users were soon tweeting about the irony of police using the phrase while appealing for calm and failing to confirm claims that the man had a bomb strapped to his body.

As the number of comments rose, a message appeared on the official Hertfordshire police Twitter feed: "We are using the hashtag #watfordbomb as it was created by users and we want to make sure our messages are seen and to keep you updated."

Police forces around the country are increasingly turning to Twitter and other social media sites to keep people informed about fast-moving events. Before the TUC demonstration in March, Metropolitan police officers announced they would use Twitter to warn protesters who were about to be "kettled", and to counter any false rumours. The Met's assistant commissioner, Lynne Owens, explained that her force was refining its practices in an effort to engage with protesters "in real time" and respond to concerns over policing, broadcasting warnings or quashing rumours while events were taking place.

Ministry of Defence bomb disposal experts were drafted in to assist with the Watford siege, which lasted three and a half hours and sparked panic in the area.

Jeffrey Doctors, 65, senior partner at nearby law firm Jeffrey Doctors & Marchant, described hearing gunshots ring out across the town at about 1.25pm. Minutes later, a man was led out of the bank and arrested by three police officers, he added.

"We thought we heard gunfire. It sounded like several gun shots but we don't know," he said. "It was several loud bangs. Then we saw a white man in his 30s being led out of the bank. It was all very frightening.

"Police were outside in the street with guns. There were a lot of them."

Witnesses also described seeing police pulling people back from the bank as helicopters flew overhead.

Others described the fear that gripped the public when the area was evacuated by police during the incident, which disrupted traffic and affected hundreds of people in the town centre.

Ryan Saunders, 22, who was heading to work at BHS at the time, said: "There were lots of people panicking and then I saw the bomb disposal van speed past me. Everyone was desperate to get away."A Hertfordshire police spokeswoman said: "An explosive device of some kind – as yet we don't know what this is – has been recovered. The suspect is being taken into police custody for further questioning."

"No members of the public have been harmed, however there was considerable disruption in the town centre."Chief Inspector Mike Pryce praised the "real community spirit and support from the public", and thanked all who had been inconvenienced by the "unusual events".