London (CNN) US President Donald Trump was accused of behaving "like a petulant child," during a debate in the British parliament about the controversial decision to invite him on a state visit to the UK so early in his term.

The comment arose as members of Parliament were discussing a petition calling for the trip to be downgraded because of Trump's "well-documented misogyny and vulgarity."

Opening the largely symbolic debate in Westminster Hall, Labour MP Paul Flynn said: "We all in this room hold in great respect the US presidency, their constitution, their history," and that there was "no question of any disrespect towards that country."

But Flynn -- who described Trump's intellectual capacity as "protozoan" -- argued that to grant the honor of a state visit to a president who had acted "like a petulant child" would send the message that Britain approved of his behavior and comments.

Labour MP David Lammy said it was to be expected that the US President would be invited to the UK, but that offering Trump a full state visit after only seven days in power was not acceptable.

He said that for the country to make such an invitation was to "abandon all its principles," and suggested that the move came because the government was desperate to make trade deals.

"We didn't do this for Kennedy, we didn't do this for Truman, we didn't do this for Reagan, but for this man ... we say, 'Please come and we will lay on everything, because we are so desperate for your company,'" he said. "I think this country is greater than that."

Just spoke against Trump's state visit. This country is better than this. My children, especially my daughter, deserve better than this. pic.twitter.com/v52ndYSqMc — David Lammy (@DavidLammy) February 20, 2017

Conservative MP Simon Burns argued that the visit should go ahead -- whatever people think of Trump -- because of the close links between the US and the UK.

"America is our greatest ally. It has been for a considerable amount of time," he said. "It has stood by us, shoulder to shoulder, in our hour of need, as we did in their hour of need, particularly during 9/11."

He added, "So it is to my mind foolish to allow our personal views and assessments of an individual, and some of their more grotesque characteristics and behavior, to blur what is in Britain's national interest."

Petition signed by 1.8m

Fellow Conservative MP Crispin Blunt said withdrawing the invitation risked embarrassing the Queen, but that the trip could simply be postponed. He suggested that 2020 -- the 400th anniversary of the Pilgrims' journey to America -- would be an appropriate moment for a state visit.

MPs held the session after the petition attracted more than 1.8 million signatures , easily crossing the 100,000 threshold qualifying it for a parliamentary debate. A rival 310,000-strong petition in support of the state visit was also being discussed.

The Speaker of the House of Commons, John Bercow, faced calls for his resignation from some MPs after he said he was "strongly opposed" to letting Trump address lawmakers during his visit because of Parliament's "opposition to racism and sexism."

Bercow is one of three parliamentary officials who must approve any invitation for someone to speak in Westminster Hall, the venue typically used for grand occasions of state, where Monday's debate is being held.

Protests outside parliament

Controversy over Trump's visit intensified after he imposed a travel ban -- later blocked by courts in the US -- on nationals from seven mainly Muslim countries.

Hundreds of people gathered outside the Houses of Parliament to protest the US President's visit, according to The Press Association.

Scottish National Party MP Patrick Grady said noise from the demonstration could be heard inside the hall.

Noise from #StopTrump demo can be heard in Wmin Hall as #TrumpPetition debate continues. While business in main chamber finishing early. — Patrick Grady (@GradySNP) February 20, 2017

The Stop Trump Coalition had earlier said it expected up to 20,000 people to take part. Smaller protests in support of migrants were also due to take place around the country.

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According to the Guardian newspaper, options under consideration include inviting Trump in August when Parliament is in recess -- allowing the government to sidestep the controversy over whether the President should address lawmakers. That would also allow the Queen to host Trump at her summer retreat in the north of Scotland, Balmoral Castle, rather than the more high-profile Buckingham Palace in London.

If Trump's trip was downgraded to an "official visit," the President would be deprived of a ride in the Queen's carriage, gun salutes, a banquet at Buckingham Palace and other ceremonial honors typically associated with full state visits.

U.S. President Barack Obama and Queen Elizabeth II during a banquet at Buckingham Palace in 2011.

No US President has received a state visit in his first year in office.

President Barack Obama was afforded the honor 28 months into his tenure, while George W. Bush was extended the invite after 32 months at the White House.