Former US Vice-President Mike Pence has testified as part of a criminal investigation into alleged efforts by Donald Trump to overturn his defeat in the 2020 election.

Mr Pence, 63, sat for more than seven hours before a federal grand jury in Washington DC, sources told the BBC's US partner CBS News.

He was issued with a subpoena to testify under oath earlier this year.

The questioning by prosecutors took place behind closed doors.

His appearance on Thursday came just hours after an appeals court rejected a last-ditch bid by Mr Trump's legal team to stop Mr Pence from testifying.

Mr Pence's lawyers had also sought unsuccessfully to challenge the subpoena, arguing that his role as president of the Senate during his time in office meant he had congressional immunity.

His eventual testimony, which had been sought for months, is a major milestone in the two-year investigation which is being led by special counsel Jack Smith, a former war crimes prosecutor who was appointed to the role by Attorney General Merrick Garland.

The investigation has been gathering evidence about whether Mr Trump and his allies broke federal law in their efforts to challenge the result of the 2020 election, which was won by President Joe Biden.

It is also investigating the US Capitol riot on 6 January 2021, when Mr Trump's supporters stormed the building in an effort to prevent the election result from being certified.

Mr Pence, who like all vice-presidents was also president of the Senate - a mostly ceremonial role - could in theory have derailed the final certification of the election result and delayed the transfer of power.

Mr Trump publicly pressured his vice-president to do so, and his refusal led him to lash out at Mr Pence.

Trump supporters then chanted "hang Mike Pence" as they stormed Congress and marauded through the corridors of the Capitol building as politicians, including Mr Pence, sheltered inside.

Mr Pence is considered a key witness in the investigation and, while it is not immediately clear what he told the grand jury, prosecutors will likely have asked him about his interactions with Mr Trump and his team in the days and weeks leading up to the riot.

"We'll obey the law, we'll tell the truth," Mr Pence said in an interview with CBS on Sunday. "The story that I've been telling the American people all across the country... that'll be the story I tell in that setting."

Mr Pence has spoken publicly about the Capitol riot and the pressure he faced to challenge the election result. "President Trump is wrong. I had no right to overturn the election," he said in a speech in February.

In his memoir, So Help Me God, Mr Pence wrote that Mr Trump had attempted to pressure him into blocking the certification of the election result on the morning of the riot. "You'll go down as a wimp," the then-president apparently told Mr Pence.

He has also accused Mr Trump of endangering his family as well as others who were at the Capitol, saying history will hold him "accountable".

Mr Pence is reportedly considering a presidential bid of his own in 2024, which would see him challenge his former boss directly for the Republican nomination.

Mr Trump, who has already launched his bid to return to the White House, was in New Hampshire on Thursday for a campaign event. When asked by NBC News about Mr Pence's testimony, he commented: "I don't know what he said, but I have a lot of confidence in him."

The former president is facing other legal issues, including another federal investigation led by Mr Smith into the potential mishandling of classified documents.

There is also a separate investigation in Georgia into alleged efforts to overturn the 2020 election result.