* Vatican offers excommunicated Catholics return to the fold
* Mass confessions to take place in central Madrid park
MADRID, Aug 17 (Reuters) - The Vatican has granted priests the right to forgive the sin of abortion when hearing the confessions of hundreds of thousands of young people attending a Roman Catholic youth festival in Spain this week.
The termination of pregnancy is a sin punishable by excommunication under Church law. The World Youth Day (WYD) pilgrims will attend a mass confession in the presence of Pope Benedict on Saturday in a central Madrid park.
"This (concession) is to make it easier for the faithful who attend the World Youth Day celebrations to obtain the fruits of divine grace," the Madrid archdiocese said in a statement on its website.
Two hundred white portable confessional cabins have been erected in Madrid's Retiro Park where hundreds of priests will take confessions in different languages from the pilgrims who have travelled to Spain from around the world.
The pontiff will sit in one of the booths on Saturday morning to hear confessions from three visitors, ahead of a mass with up to 6,000 seminarians.
The Vatican already announced on Aug. 11 that it had authorised a plenary, or full indulgence, to all the young people attending the celebrations.
An indulgence is a remission of the temporal punishment a person is due for sins that have been forgiven and is traditionally granted to WYD pilgrims.
Pope Benedict will arrive in Spain's capital on Thursday for a four-day visit culminating in a mass on Sunday in the Cuatro Vientos aerodrome which over two million people are expected to attend.
The cost of the papal visit has already sparked heavy criticism at a time of economic hardship in Spain from disaffected youths, as well as gay and lesbian and transgender groups, atheists and even some Spanish priests.
A protest march has been organised for Wednesday in central Madrid.
Spanish Prime Minister Jose Luis Rodriguez Zapatero, an open secularist, has clashed with the church over abortion and has changed the law during his tenure to make it easier for women seeking to terminate unwanted pregnancies. (Reporting by Judy MacInnes; additional reporting by Tom Heneghan, editing by Rosalind Russell)