The demonstration was part of a fierce backlash in the UK to the announcement last week by Prime Minister Theresa May that US President Donald Trump had accepted an invitation conveyed from the Queen for a state visit later this year.
The number of signatories on a petition calling for Trump's state visit
to be cancelled has soared past 1.7 million -- and the issue will now be debated in Parliamen
t. Among those to have signed it are Jeremy Corbyn, leader of the opposition Labour party, and Ruth Davidson, the leader of May's Conservative party in Scotland.
Anger has intensified over Trump's decision to impose a 90-day ban on travellers from seven Muslim-majority nations and to restrict refugee arrivals
-- enacted in an executive order signed hours after May left the White House.
All this has left the Queen at the centre of a political firestorm -- not a comfortable place for a constitutional monarch.
In a letter to Tuesday's Times newspaper, the respected former head of the Foreign Office, Peter Ricketts, said May had put The Queen in a "very difficult position" and should protect her by downgrading Trump's invitation to an "official visit". Such a trip would be shorn of the ceremonial hoop-la that The President is currently expecting, involving only talks with the Prime Minister and a low-profile courtesy call on the monarch.
May is in a precarious position. If she bows to pressure and downgrades the trip she risks more than losing face: it could cost her a crucial trade deal she's been working on with President Trump.
It could also test relations with Buckingham Palace: Ricketts noted in his letter to the Times that the government's role in protecting the monarch from political controversy was being "put under strain because of the ill-urged advice to the Queen to rush out an invitation to President Trump".
As Ricketts' letter made clear, the Queen doesn't actually have a say on who is invited on a state visit to Britain. Instead, she acts on the advice of the Foreign Office in consultation with Downing Street. May has the ultimate sign-off and only she can alter or withdraw the invitation.
So far the Prime Minister is resolute: the invitation stands, she said at a press conference with the Irish prime minister on Monday evening.
State visits are a powerful diplomatic tool for British governments and they are used unashamedly. Who can resist a sleepover and state banquet at Buckingham Palace, hosted by the Queen, with the pomp and pageantry of a horse-drawn carriage procession thrown in? If anyone can out-bling Donald Trump, it's Elizabeth Windsor -- and we already know Trump's mother was a fan
Whether The Queen is a fan of the President is a different matter and, not to put too fine a point on it, irrelevant. Her role is to stay above politics and over the decades she has hosted all sorts of controversial figures from Nicolae Ceaucescu of Romania to Robert Mugabe of Zimbabwe. Never have we heard a public utterance of her personal feelings about any of her official guests -- she's a stickler for protocol.
The government has its own reasons for wanting to improve relations with the guests, and whether or not the Queen relishes the idea of hosting President Trump, she will oblige, as is her duty.
We don't know how Trump would react to a downgraded invitation but he's unlikely to welcome it and at worst it could be taken as an insult and cause a diplomatic rift.
The Queen no doubt eagerly awaits the guidance from her ministers.