Pope Benedict XVI has ushered in the New Year with a plea for peace for the whole "human family" in a world marked by the "dark shadows" of war, violence, poverty and destruction of the environment.

In his homily at New Year's Day mass at St Peter's Basilica he said that the traditional family was the foundation of world peace. He added: "We all aspire to live in peace, but real peace is not the simple conquest of man or the result of political agreements: it is above all a divine gift."

In an indirect but clear attack on civil unions and gay marriages the Pope said "The family is the primary agent of peace, and the negation or even restriction of rights of the family threatens the very foundations of peace." In Spain, one of a number of European countries where gay marriage is sanctioned, hundreds of thousands marched in a "Family Day" demonstration last Sunday to defend traditional family values.

"Everything that serves to weaken the family based on the marriage of a man and a woman, everything that directly or indirectly stands in the way of its openness to the responsible acceptance of a new life, everything that obstructs its right to be primarily responsible for the education of its children, constitutes an obstacle on the road to peace," the Pope said.

In his Angelus address, delivered after mass from his window high above St Peters Square the Pope told thousands of pilgrims gathered on a cold but bright sunny day that "We have begun a new year, and I hope it will prove a serene and profitable one for all".

In a written message marking the fortieth anniversary of World Peace Day, celebrated by the Catholic Church on January 1 since the days of Pope Paul VI, the pontiff said that "In a healthy family life we experience some of the fundamental elements of peace - justice and love between brothers and sisters; the role of authority expressed by parents; loving concern for the members who are weaker because of youth, sickness or old age; mutual help in the necessities of life; readiness to accept others and, if necessary, to forgive them".

However humanity was "unfortunately experiencing great divisions and sharp conflicts which cast dark shadows on its future," the Pope said. War and violence, exploitation of the weak, poverty and underdevelopment, destruction of the environment and the nuclear arms race were all signs that nations had still not learned to live together in harmony.

In an apparent reference to Iran he said that "the danger of an increase in the number of countries possessing nuclear weapons causes well-founded apprehension in every responsible person". He also referred to civil wars in Africa, and the "spiral of violence" in the Middle East.

Pope Benedict said the costs to protect the environment must be shared globally, but - as in a family - with an awareness of the limited resources of the poorer nations and the greater responsibility of the richer nations. People around the world, again like members of a family, must adhere to common values "fostering true freedom rather than blind caprice and protecting the weak from oppression by the strong."

In his New Year's Eve homily on Monday the Pope warned that because of a "deficit of hope" young people in the Western world were seeking sexual gratification and "profanation of the body" at the expense of spiritual and family values. "This constitutes the dark evil of modern Western society" he said. "Even in Rome you can feel a lack of hope and trust in life." Western society risked an "educational emergency" if it did not teach its children a moral code.