Joe Biden has denounced disruptive demonstrations by US President Donald Trump's supporters after they swarmed a Democratic campaign bus in Texas.
The FBI says it is investigating the Friday incident in which a convoy of vehicles flying flags in support of Trump's re-election bid surrounded a bus carrying campaign staff for the Democratic challenger on a highway between San Antonio and Austin.
The incident, which was captured on video that was retweeted by Trump on Saturday with the message, "I LOVE TEXAS!", prompted the Biden campaign to cancel at least two of its Texas events as Democrats accused the president of encouraging supporters to engage in acts of intimidation.
"We've never had anything like this. At least we've never had a president who thinks it's a good thing," Biden said while campaigning in Philadelphia on Sunday.
He also referenced reports that Trump supporters had shut down a major roadway in New Jersey.
Video showed a swarm of pick-up trucks and SUVs bearing pro-Trump flags surrounding the Biden campaign bus on Interstate 35 in Texas.
The Biden campaign said the Trump caravan tried to force the bus to slow down and to run it off the road.
One video clip aired on CNN showed a Trump-flagged pick-up swerve into the side of another vehicle travelling just behind the bus, which was reportedly being driven by a Biden campaign staffer.
The campaign said staff called 911 and local law enforcement assisted the bus in reaching its destination.
Neither Biden nor his running mate, Senator Kamala Harris, was aboard the bus.
"FBI San Antonio is aware of the incident and investigating," an FBI spokeswoman told Reuters.
The confrontation came as polls showed an unexpectedly tight race between Biden and Trump in Texas, which has long been a Republican stronghold.
Video on social media on Sunday showed vehicles flying pro-Trump flags also blocking traffic on bridges in New York and New Jersey.