Paul, a 34-year-old acquaintance of mine, came home from work one day last summer to find that he had lost everything. His wife had found a new boyfriend, he was thrown out of the family house and his three children were prevented from seeing him for five months.

When the divorce case came to court he was told that his wife would be keeping the house, he would have to pay maintenance amounting to 75% of his earnings and he would be allowed to see his children only once a month. He has no record of violence or criminal behaviour, but his former wife still refuses to allow more access to their children.

After living in a rented room for nine months, Paul was able to buy a one-bedroom house in Essex. He is now recovering from a nervous breakdown.

On Friday more than 1,000 angry fathers fighting for the right to see their children took to the streets in London because of experiences like Paul’s. A group of up to 50 stormed court one of the Family Division of the High Court in High Holborn, bringing proceedings to a halt. Today demonstrators in Glasgow are planning to mark Father’s Day with another equally strong protest.

Fatherhood is being radicalised. For decades Father’s Day has been seen as little more than a commercial opportunity for card and confectionery manufacturers. But for the “forgotten fathers”, who increasingly see themselves as victims of the widespread breakdown of marriage, today has a bitter taste.

Fathers 4 Justice (F4J), a group fighting for the civil rights of divorced fathers, is behind the court protests. The organisation has a membership of more than 1,200 and says it has been growing at a rate of 100 a week. Fathers, grandparents and even some mothers have joined its demonstrations, picketing courts in the south of England.

In December, 200 fathers dressed as Father Christmas sat in the lobby of the lord chancellor’s department. Next month four F4J members will go on trial charged with painting the doors of court welfare offices purple (the international colour of equality).

F4J is demanding a change in the way family law is administered.

It argues that the law is automatically stacked against men, and its particular target is the secrecy of child custody proceedings, which leaves fathers unable to draw public attention to what they perceive to be miscarriages of justice.

Matt O’Connor, F4J’s co-founder, talks of “opening up the courts to the general public” in order to break the “conspiracy of silence”. Many divorced fathers are angry with the legal system, he says, and “sooner or later a father is going to pick up a gun and go visit a judge”.

At the heart of the fight is the law’s perception of a father’s role in the life of a family. It sees him in the archaic function of a detached breadwinner who plays little or no part in the raising of his children. In custody disputes the 1989 Children Act favours the “primary carer” and courts almost invariably cast the mother in this role.

Dame Elizabeth Butler-Sloss, president of the Family Division, has recognised that 40% of fathers lose contact with their children within two years of a relationship breaking down. This suggests that an estimated 100 children lose contact with their fathers in Britain every day. F4J argues that this is the result not of indifference on the part of the fathers but of the obstacles placed in their way.

In 2001, family courts made more than 55,000 contact orders affecting 80,000 children, in most cases ordering mothers to allow the father regular contact with their children. Nearly half were flouted, yet the courts did not enforce them because judges hesitate to impose either fines or jail sentences on mothers. Alternative punishments of community service or transfer of custody are rarely considered.