LONDON — It was a warm, drizzly Saturday night, and the bars at Borough Market here were packed, as usual. Tucked below an offramp of London Bridge, near the banks of the Thames, the market is a warren of narrow streets populated with trendy bars and restaurants, where some people had just finished watching the Champions League soccer final.

Then came the loud bang of a crashing van. Then came three men slashing at patrons with knives. Then screams, sirens, chaos — and, finally, gunfire as arriving police officers shot and killed the three assailants.

“A huge group of people started running towards us,” said Graham Forester, who was in a taxi arriving at Borough Market as the attack unfolded. “One woman screamed at my cabby, telling him to turn around. One guy fell over really badly, while he was running, and I saw a pool of blood on the pavement.