The mood in Liverpool had been jubilant. Paul O’Brien and his family had flown in from Ireland to celebrate their soccer team’s Premier League title. “Everyone’s singing all day, everyone having a good time, everyone talking to one another,” Mr. O’Brien recalled.

“And then…”

Standing along Water Street in the heart of Liverpool on Monday, Mr. O’Brien, his son and his parents suddenly found themselves being shoved, hard, by a surging crowd. Out of nowhere, a dark-color car was barreling down a street jammed with pedestrians. Had it not been for the people pushing them out of the way, Mr. O’Brien said on Tuesday, they might have been in the car’s path.

“We didn’t realize until we were pushed,” Mr. O’Brien said. “And then we actually realized, ‘Oh, something went on here.’ There were people lying on the ground. But the people of Liverpool were amazing. Those people were very brave to push kids and people out of the way.”

In an instant, the joyful day had turned dark. At around 6 p.m. on Monday, police said, a 53-year-old British man plowed his vehicle into the crowd, injuring almost 50 people, including 27 who were sent to one of several hospitals in the area. Two people, including a child, were seriously injured. A fire department spokesman said that four people had to be pulled from underneath the car.

The impact of the vehicle slamming into the sea of pedestrians quickly rippled through this proudly working class city on England’s northwestern coast.