LONDON — It was midafternoon when Mike Finnerty, who sells cheese at his Borough Market shop just south of London Bridge, realized that something was wrong. An unusual flow of people had suddenly gathered in front of his stall, he said, and they seemed “alarmed.”

What he did not know was that a man dressed in black and armed with knives had gone on a murderous rampage in a grand meeting venue called Fishmongers’ Hall on the opposite side of the bridge, just north of the Thames River. But Mr. Finnerty sensed the danger on Friday, he would later tell the BBC and write on Twitter.

So he and another employee rushed some customers — a couple from Vancouver and a young American man — into a cheese refrigerator and locked the door. Then he called the police.

The “operator said it was an attack and not to move,” he wrote. He said he could hear shouting outside the door, but he and the group huddled together in “pretty close quarters.”