It was in a dingy socialist meeting hall a century ago in Copenhagen that women from 17 countries gathered and launched the idea of a day which would champion the rights of women. All over the world this weekend women are marching, celebrating and protesting, not least in London where last night thousands of people thronged Trafalgar Square to mark the 100th International Women's Day.

The theme chosen this year is progress: the progress women have made in the past century, and the long journey that many have ahead of them. The latest statistics on the lot of women in Britain and around the world suggest that some undoubted gains over those 100 years have now stalled, or been reversed, more recently.

Just 19.5 per cent of the MPs in Britain are women; a record so poor that it puts the UK 69th in the world for our proportion of female parliamentarians – behind Afghanistan, the United Arab Emirates and Pakistan. Of course, 100 years ago women had no vote and would wait almost another decade to get a single MP with no Y chromosome, but equality is further off than it might appear. According to a hard-hitting report by the Fawcett Society to be published tomorrow, at the current rate of progress it will take 200 years to achieve an equal number of women in Parliament.

When it comes to implementing the laws of the land, women have even less say than in Westminster – and are now losing what little input they had. In 2008/09, the number of women applying for Queen's Counsel was at its lowest level in 10 years. Higher up, there is only one female judge on the UK Supreme Court and just 15 of 109 High Court judges are female.

The picture is not much better for those at the other end of the legal system: two-thirds of the women in prison are there for non-violent crime, compared to 45 per cent of men; and since 1997 the female prison population has soared – increasing by 60 per cent, as opposed to a 28 per cent rise for men.

Although women's freedoms in Britain are clearly manifest and to be celebrated, some women have yet to benefit. The broadcaster and parliamentary candidate Esther Rantzen says: "There are still women in this country who are forced into marriages, very subservient to the men in their families – I'm told there are women who are told how to vote by men. I'm very aware that the freedoms I was brought up to prize – equality of education, equality of ambition – aren't available to all the women in the UK."

There are also reasons to be troubled by the numbers exposed to violence. Some three million women in the UK undergo rape, domestic violence, trafficking, forced marriage and other violence every year. Twenty per cent of people still believe it is sometimes acceptable for a man to hit or slap his girlfriend if she is wearing revealing clothes in public.

But one cause for concern is more intangible: how women are perceived and how they see themselves. Natasha Walter, author of The New Feminism, is worried: "The eagerness for change has slowed. I think we've slowed down because of complacency: there was a feeling that the argument's been won and we've got the policies in place. Also, there's been a cultural change resulting from the mainstreaming of the sex industry, which has narrowed the options of young women as to what being attractive is."

All this is arguably a side issue for the five million or so women living in poverty in Britain. Women have 40 per cent more chance of being poor than men, with the gender pay gap still at 16.4 per cent for full-time work and 35 per cent for part-time.

The figures from the Fawcett Society suggest that Britain has some way to go before its society can be considered equal. Ceri Goddard, the Fawcett's chief executive, says: "Since the first International Women's Day, the feminist movement has achieved some pretty totemic successes – the right to vote, an equal pay act, and more access to education and work. But, for all the strides we've made, many of our successes are fragile: for example, after the increases of 1997 we might well end up with fewer female MPs this time. It's clear that we still need a major push to get women's equality away from the margins and into the centre of the key debates."

Campaigners yesterday highlighted the universal challenges faced by women around the world. Of course, the hardship and challenges faced by women in Britain can seem almost insignificant when compared to that tackled daily by those living in countries blighted by poverty, oppressive regimes and institutionalised misogyny. In the developing world, access to education, proper health care and basic freedoms can be forever blocked if you are born female. On the following page, the IoS has highlighted six personal experiences that illustrate some of these of these issues.

Ellie Levenson, author of The Noughtie Girl's Guide to Feminism, says the solution lies in choosing the right battles. "Internationally, there are huge issues about women being denied basic human rights – from female genital mutilation, which also happens here, to women not being allowed to go out on their own, being denied passports or not allowed to drive. Domestically, we have fights for equal pay in the workplace. In the home, while men do more child care, there are other areas such as care for the elderly and even cleaning."

The charity Concern, whose Women Can't Wait campaign will be launched tomorrow, draws attention to the fact that, for the first time in human history, there are more than a billion people going to bed hungry every night – and the majority of these are women. Phoebe Asiyo, a women's rights advocate in Kenya who is now the UN Development Fund for Women's goodwill ambassador, said: "Women are still more likely than men to be at risk of hunger because of systematic discrimination. It is unacceptable that though poor women produce the majority of food, they make up the majority of the world's hungry."

But there are reasons to be optimistic. In Britain, though the pay gap persists, there are signs that it is closing. Women's median weekly earnings for full-time employment rose by 3.4 per cent between 2008 and 2009, men's rose by just 1.8 per cent. In health care, life expectancy for women in Britain continues to outstrip men's. Opinions are changing, too. A survey for the Government's Equalities Office, to be released tomorrow, shows that 63 per cent of Britons believe there are too few women in Parliament.

Worldwide, some 39 million girls are denied even a primary education, but in the UK girls consistently outperform boys at school.

So are things still improving for women? That is certainly what the historian Lisa Jardine believes. "Britain loves to think things are slipping back, but things are systematically improving for women – it's just that we expect more. Women's expectations will stop being 'realistic' when they reach absolute parity with men. I don't know when it will happen, but it will happen."

Million Women Rise: Thousands join the march through the streets of London

If the theme was oppression, the mood was anything but. Whistles, songs, chants and cheers drowned out the regular din of a Saturday lunchtime on Oxford Street as thousands of women rejoiced in their collective – if temporary – power. Men on the pavement watched open-mouthed, unsure what to make of the colourful spectacle of yesterday's Million Women Rise march in London.

Monique Stretton could have told them. The 21-year-old from Leicester says that, for her, turning up was all about showing any of the three million women who experience violence every year in the UK they are not alone. "Hopefully, people who need help and who walk past will realise there is support out there and make that phone call," she says.

All along the route from Speakers' Corner in Hyde Park to Trafalgar Square, people are transfixed by the display of female solidarity. Camera phones flash as passers-by capture the moment. Bizarrely, it's mainly men who are taking pictures. "It's always the men," says Amy White, 24. "They take them while clutching on to their girlfriends."

Marchers like Amy and Monique embody the latest wave of women who are standing up for their rights, specifically in this instance not to be abused. They are young and passionate about their cause. "It's events like this that make people realise there is a feminist movement. It's celebratory, not angry," Amy adds.

For Sabrina Qureshi, who started the marches three years ago, the events are about raising awareness of violence against women. "I'd just had enough. A young woman I used to work with saw a man attack a woman in the street and she felt really powerless to do anything. So we decided to march to increase our visibility and to show that there is a way forward, a shared vision of a world without violence."

Qureshi adds they are helping to empower a whole new generation. "My three-year-old niece, who has been on all three marches, now calls herself a 'super she-ro feminist'." What she made of the event is not clear but four-year-old Zayna, who came with her mum, Syreeta Loney, was definitely impressed. "It's very big." Which, one hopes, sums up the impact it will have had.

Susie Mesure

'Parity with men will happen...'

It's clear women can do anything boys can do. What's disappointing is men aren't interested in doing everything women can do. Why isn't every union campaigning for men's rights to equivalent paternity leave?

Bea Campbell, Feminist and Green Party candidate

Things are systematically improving for women – it's just we expect more. Women's expect- ations will stop being 'realistic' when they reach parity with men. It will happen.

Lisa Jardine, Professor of Renaissance Studies at Queen Mary, University of London

All over the world women are raped, buried under veils or stoned to death. Middle-class women here have got it made. They should be standing up for women without a voice.

Claire Rayner, Agony aunt and vice-president of the British Humanist Association

We are at the stage of reassessing: women's rights are becoming women's choices. A lot was done quickly, heads down. It's time we got our heads up, looked round and made choices.

Jane Robinson, Social historian

There are huge issues about women being denied basic human rights internationally – from female genital mutilation, which also happens here, to women not being allowed to go out on their own.