Gianyar, Indonesia | Thousands of men around the world are to be sterilised in what organisers dubbed a global "vasectomy-athon", to encourage men to take a bigger role in family planning and combat resistance to the procedure.

Some 750 doctors in 25 countries are to perform the procedure on more than 3,000 volunteers to mark World Vasectomy Day, with many operations being provided for free or at discounted rates.

"In helping to shoulder responsibility for family planning, men become heroes to their partners, to their families and to our future," said event co-founder Jonathan Stack.

The event is being held as a report from campaigners and donors warned efforts to get modern contraceptives to women in some of the world's poorest countries are not on track, with millions fewer reached than had been hoped.

At a ceremony in a temple on the Indonesian island of Bali, the headquarters for World Vasectomy Day this year, the first six men to undergo the procedure were presented to an audience before being taken outside to mobile health clinics to be sterilised. 

The men lay on an operating table in the clinics -- buses fitted out with medical equipment -- while doctors performed the short procedure, which involves cutting the tubes which transport sperm from the testicles, under a local anaesthetic.

Vasectomies were also being carried out to mark the day in countries including India, the United States and Spain.

Around four in 10 pregnancies worldwide are unplanned and event organisers said that family planning is still too often left to women, who are the ones who must deal with the consequences of unintended pregnancies.

In many countries, less than one percent of men get vasectomies, despite the fact the procedure is safe and in the majority of cases has no effect on sex life, the organisers said.

- Worldwide resistance -In Muslim-majority Indonesia, efforts to persuade men to get vasectomies have been hampered after the country's top Islamic clerical body several years ago declared the procedure "haram", or against Islamic law.

Other attempts to encourage vasectomies have backfired. A district on Sumatra announced in 2012 it would hand out cash to civil servants who underwent the procedure -- only for the move to spark anger from women who feared their sterilised husbands would have affairs.

Elsewhere around the world the procedure is burdened by controversies, and in many countries campaigners have to overcome the misguided belief that it impairs a man's virility.

Iran recently eliminated free vasectomies, as it seeks to improve its birth rate, and there has even been resistance from experts in sub-Saharan Africa, who have expressed concern that widespread use of vasectomy would lead to lower usage of condoms and so higher HIV rates.

Prominent vasectomy doctor Doug Stein, who has performed the procedure on more than 30,000 men and founded World Vasectomy Day with Stack, told the Bali audience that the operation was positive for men, their families and societies.

"It seems to be a wonderful option for men who have had as many children as they want," he said.

Friday's event was the third World Vasectomy Day, with the first held in 2013 and headquartered in Australia. 

Organisers chose to base this year's event in Bali to coincide with an international family planning conference that had been due to take place on the island, but which was postponed after volcanic ash closed Bali airport for days.

A report that had been intended for release at the Bali event on Friday showed that over 24 million more women and girls in poor countries now have access to contraceptives since a 2012 commitment by donors and campaigners to make them more available. 

But this is 10 million fewer women than had been hoped, according to the progress report.