There's quite a view from the chamber in City Hall where the London assembly meets to scrutinise the mayor. Occasionally during one of Boris Johnson's more vague and waffling answers at mayor's question time, my eyes wander to admire the fine vista of the north side of the Thames, and come to rest upon one of London's most ancient buildings – the Tower of London.

The tower was, of course, the place where enemies of the monarch were once imprisoned. Judging from the reaction from some sections of the media to a short blog post I wrote bemoaning the reaction of some politicians to the royal christening, there are some who'd like the tower to be bought back into use for this very purpose.

The story originally ran in the Evening Standard. Unsurprisingly, it caused the Daily Mail to froth over with faux-outrage. Conservative MP Bob Neill accused me of being "out of touch", proving once again that no one fawns over royalty quite like a Tory. Still, it's nice to see Neill standing up for those who can't stand up for themselves. The Huffington Post ran the story as the main headline on its front page, asking readers if they agreed with me.

The media reaction demonstrates once again how monarchy infantilises us by creating a semi-mystical institution that is supposedly above criticism. The expectation is that we should suspend our critical faculties, nod, smile and say how bloody marvellous the whole thing is. No thanks. Opinion polling consistently shows that 20-25% of the British people want a democratic alternative to hereditary monarchy. That's millions of people who deserve to have their voices heard.

The Queen is quite understandably held in high public affection. But that does not mean that we should not debate the serious constitutional issue of whether it is right that in the 21st century we continue to select our head of state by accident of birth rather than by universal suffrage. This is not some minor issue. Through royal prerogative, powers are exercised by the prime minister on behalf of the monarch with absolutely no oversight or scrutiny from parliament. Perhaps the most insidious of these is the ability it gives a prime minister to pack the unelected House of Lords with his stooges, a practice that has always gone on but which has been taken to new extremes by David Cameron.

The royal family is granted absolute protection from public scrutiny in a manner that would put the Daily Mail into apoplexy if it were any other part of the public sector. Most egregiously, the royal family is entirely exempted from Freedom of Information requests. It is well known that Prince Charles uses his position to lobby ministers, in particular when it comes to his pet issues of architecture and homeopathic medicine. Yet ministers have vetoed all attempts to bring correspondence between Charles and ministers to light. This is hardly transparent government.

Members of parliament were quite rightly lambasted in the media when details of abuses of expenses were revealed. Yet we don't hear a peep from the Daily Mail about extravagant royal expenses. To quote Ian Davidson MP, a former member of the public accounts committee, "every time we examined royal finances we found extravagance and indulgence as well as abuse of expenses by junior royals".

Some people thought that my comments about the royal christening were somewhat churlish. So let me put this into context. I represent a city in which 650,000 children live in poverty, where 391,000 children live in overcrowded housing and 17,000 live in severely overcrowded housing. In the sixth richest city in the world these numbers should shame us all. You'll have to forgive me for being angry that valuable parliamentary time was spent paying tribute to the christening of a very rich, wealthy baby.

When mild criticism of our hereditary monarchy provokes such a storm in the media, it's no wonder that politicians tend to avoid straying into the debate. But we shouldn't be cowed. On Saturday I spoke at the annual general meeting of Republic, the organisation that campaigns for an elected head of state, where we also heard from other European republican movements. The Swedish movement boasts three MPs from three different parties on its management committee, something that I would hope to see replicated in this country in the near future. Republicans everywhere should speak up, be proud of what we believe in and work together to win the argument for a democratic alternative to monarchy.