Diners at The Hub on Thursday afternoon were shocked by the arrival of armed police.
One man was led away in handcuffs by officers, who were carrying the man's crutches. The incident happened at around 2pm.
An eyewitness told us: "It didn't register what was happening at first. There were several armed police officers, and a police dog, and I thought maybe they were on a training exercise.
"Then I saw them leading away the handcuffed man and realised it must be something more serious."
He added: "We are used to seeing occasional armed police nowadays, but nothing on this scale in such a pleasant area.
"It must have been something really serious for so many officers to be involved."
Thames Valley Police have been contacted for more information.