Leading Italian Catholic politicians have defied a call by the Vatican to boycott a two-day referendum on assisted procreation.
A 50% turnout is needed for the vote to be valid
They included President Carlo Azeglio Ciampi who was among the first to vote when the polls opened on Sunday.
Mr Ciampi and his wife, both staunch Catholics, are among the most popular of Italian politicians.
Opposition leader Romano Prodi and Deputy Prime Minister Gianfranco Fini are also voting with their families.
The turnout in this referendum is crucial. Unless 50% of Italy's 40 million voters turn out to cast their votes, the referendum will be invalid.
About 13% had voted by 1900 local time (1700 GMT), according to partial figures released by the interior ministry.
Polls are due to close at 2200 on Sunday and reopen at 0700 on Monday.
Italians are being asked if they want to amend a restrictive law on assisted procreation which has forced many infertile couples who want children to seek medical assistance abroad
The restrictions include:
- a ban on donor sperm and eggs;
- a ban on scientific research on embryos;
- a ban on embryo screening for couples with hereditary diseases;
- the rule that only three embryos per treatment can be created, all of which have to be implanted at the same time.
The law was drafted and passed amid concerns that Italy had become one of the world's most liberal countries regarding assisted fertility.
The Pope has praised bishops for their stance on the vote
From being a country where there was virtually no regulation in bioethics - and one fertility doctor was even enabling women in their 60s to become pregnant, and promising human cloning, the law has now swung the other way, says the BBC's David Willey in Rome.
The Roman Catholic Church wants to keep it that way, our correspondent says.
Italian bishops with the backing of the newly-elected Pope Benedict XVI, have told people in the predominantly Catholic country to boycott the vote on moral grounds.
Italian cities are plastered with posters telling Catholics to stay away from the polls.
Many voters say they are angry at this interference by the Church in the democratic process and President Ciampi's lead sent a powerful message to the electorate.
Church-state relations in Italy are facing their most significant challenge since the legalisation of divorce and abortion in the 1970s, says our Rome correspondent.