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Joe Biden lost his train of thought multiple times, mumbled and froze mid-sentence in a disastrous first presidential debate against Donald Trump.

The 81-year-old president’s voice was faint as he rambled his way through and struggled to coherently answer questions.

He frequently paused to cough and looked down at his notes on the podium, rather than into the camera, as millions of Americans tuned in to watch his first encounter with his Republican rival in four years.

The debate opened with questions on the US economy, but Mr Biden repeatedly corrected his figures in lengthy responses which confused millions and billions and percentage figures.

In one particularly damaging moment, Mr Biden lost his train of thought completely and froze on stage.

His response drifted from an answer on tax policy to health policy, bringing up Covid-19 before saying, “excuse me” and trailing off again.

“Look, we finally beat Medicare,” Mr Biden said, referring to health care for over 65-year-olds as his time ran out.

Trump was quick to deliver a riposte: “That’s right, he did beat Medicaid, he beat it to death. And he’s destroying Medicare.”

After another hard to hear and rambling answer by Mr Biden, his Republican opponent quipped: “I really don’t know what he said, I don’t think he even knows what he said”.