Media playback is unsupported on your device Media caption Senator shut down and forced to go to Facebook

Democratic US Senator Elizabeth Warren was silenced by Senate Republicans on Tuesday after reading a letter written by the widow of Martin Luther King Jr.

The 30-year-old letter criticised Jeff Sessions, President Donald Trump's nominee for attorney general.

Mitch McConnell, the Republican majority leader, said Ms Warren had broken Senate rules by impugning the conduct of another senator.

She is banned from speaking again in the Senate on Mr Sessions' nomination.

The vote is expected on Wednesday.

After the rule was invoked several Democratic senators read the letter on the floor of Congress, so that it could be entered into the official record.

The incident has outraged Democrats, and delighted the Massachusetts senator's detractors.

So what is the rule that gagged her, and was it fairly applied?

How did this happen?

The incident occurred during a debate in the Senate on the nomination of Mr Sessions to be America's top prosecutor.

Elizabeth Warren started reading a 1986 letter by Coretta Scott King, written to oppose President Reagan's nomination of Mr Sessions as a federal judge.

Ms Scott King's letter alleged that Mr Sessions was unsuitable for that role because he had "used the awesome powers of his office in a shabby attempt to intimidate and frighten elderly black voters".

Ms Warren also quoted the late Senator Ted Kennedy, who called Mr Sessions a "throwback to a shameful era".

Mr Sessions' nomination process has been dogged by allegations of racism.

The Alabama senator has denied the allegations, and his supporters have pointed to his vote to extend the Voting Rights Act.

Media playback is unsupported on your device Media caption Jeff Sessions said caricature of him as a 'Southern racist was painful'

Ms Warren's reading was interrupted by the Senate's presiding officer, Republican Senator Steve Daines of Montana, who said she was breaking a rule that stops senators accusing each other of "unbecoming" conduct.

Mr McConnell, the Republican leader of the Senate, agreed.

His objection to Ms Warren's speech was put to a vote, and the chamber voted to silence Ms Warren by 49 to 43.

What is this rule?

Under Rule 19, members of the Senate are not allowed to "directly or indirectly, by any form of words impute to another Senator or to other Senators any conduct or motive unworthy or unbecoming a Senator".

Ms Warren denied breaching it, saying: "I am surprised that the words of Coretta Scott King are not suitable for debate in the United States Senate. I ask leave of the Senate to continue my remarks."

However, the Republicans disagreed, and voted to silence her.

Is the rule usually enforced?

It depends who you ask. Democrats have argued that Republicans are selectively enforcing the measure.

In July 2015 Senator Ted Cruz of Texas was not found to have violated it despite accusing Mitch McConnell - a member of his own party - of a "flat-out lie".

On 1 February, Republican Senator David Perdue tested the waters with a direct attack on Chuck Schumer, the Senate's top Democrat, who cried at a press conference on President Trump's immigration ban.

Media playback is unsupported on your device Media caption John Lewis and Cory Booker say Jeff Sessions 'hostile to civil rights'

"The minority leader's tear-jerking performance over the past weekend belongs at the Screen Actors Guild awards, not in a serious discussion of what it takes to keep America safe," he said.

Mr Schumer said of Ms Warren's treatment: "If the average American heard someone read a letter from Coretta Scott King... they would not be offended. It seems to me that we could use Rule 19 almost every day on the floor of the Senate. This is selective enforcement."

However, Mr McConnell defended the decision after the vote, saying Ms Warren had been given a warning.

"Sen Warren was giving a lengthy speech. She had appeared to violate the rule. She was warned. She was given an explanation," he said. "Nevertheless, she persisted."

What has been the reaction?

Ms Warren subsequently read Coretta Scott King's letter live on Facebook, outside the Senate.

The letter by Coretta Scott King was widely shared on social media with the hashtag #LetLizSpeak.

Mr McConnell's own words spawned a hashtag - #shepersisted - celebrating Ms Warren's perseverance.

Bernice King, the daughter of Coretta Scott King and Martin Luther King, wrote on Twitter: "Thank you @SenWarren for being the soul of the Senate during the #Sessions hearing. #LetCorettaSpeak #LetLizSpeak"

The Democratic National Committee said in a statement it was a "sad day in America when the words of Martin Luther King Jr's widow are not allowed on the floor of the United States Senate".

However, Republican Senator Orrin Hatch told NBC that Ms Warren's move was not appropriate in the chamber.

"Even if what she said was true, it wasn't the right thing to do," Mr Hatch said. "I've been appalled at the way Democrats have treated Jeff Sessions."

Why do liberals love Senator Warren?

Senator Elizabeth Warren, 67, is the darling of America's liberal left, many of whom have urged her repeatedly to run for the presidency.

Lawyer, Harvard professor, grandmother - she served in many roles before being elected as a senator for Massachusetts in 2012.

The daughter of an Oklahoma maintenance man, she is the state's first female senator, with a reputation as a crusader against Wall Street excess.

In the aftermath of the 2008 financial crisis, Ms Warren headed the Congressional Oversight Panel for the Troubled Asset Relief Program, the government bailout of the banking sector.

The Consumer Financial Protection Bureau was also her brainchild.

Many of her ardent fans might be surprised to learn this outspoken critic of the conservative establishment was a Republican until her early 40s.