* First census since 1983 marked by charges of discrimination against Rohingya
* Process extended until end of May
* Muslims say census takers refuse to register them as Rohingya
* Rights groups called for census to be postponed (Updates after census deadline extended)
By Paul Mooney
YANGON, April 11 (Reuters) - A group of women dressed in green sarong-like longyis and simple white blouses stand around a table piled with census forms, entering neat notations on spreadsheets by hand.
The women will have to go through 37,579 family census forms, according to officials, using calculators to tally the total numbers because they have no access to computers.
The scene underscores the challenges of carrying out a census in this poor and sprawling, predominantly Buddhist nation. It was supposed to end last Thursday, but was extended on Saturday until the end of May, due to "technical and logistical problems".
The census - the first in three decades - has long been mired in controversy, much of it concerning the counting of Rohingya - Muslims who live in western Rakhine state and often described by the United Nations as one of the most persecuted minorities in the world.
Officials say some 100,000 school teachers have fanned out across Myanmar on foot collecting data for the census, expected to count between 48 million and 65 million citizens.
On April 10, on what was supposed to be the final day of the census, volunteers went door-to-door in Yangon, Myanmar's commercial capital, racing to gather data for a census estimated by rights groups and other groups to cost $74 million.
Trucks with loudspeakers blared reminders for people to be counted and shops, buildings, ferries and buses were plastered with posters encouraging people to take part.
Susu Win, a volunteer tallying numbers in Yangon, said she worked 12 hours a day and interviewed, on average, 100 families.
"The biggest problem is that we had to climb eight, nine floors in four to five buildings a day with no elevators," she said.
Rights organisations and ethnic groups in Myanmar have called for the census to be postponed until it can be carried out fairly and safely.
The government had promised international sponsors that ethnic groups could choose their classification. But a day before the census kicked off, presidential spokesman Ye Htut indicated that use of the term Rohingya would be prohibited.
In Sittwe, the capital of Rakhine state, Buddhists protested against the use of the term Rohingya, saying it would give them legitimacy.
The government describes the Rohingya as Bengalis, a term that implies they are illegal immigrants from neighbouring Bangladesh. Many say they have lived in Rakhine for generations.
ROHINGYA STRESS ETHNIC ORIGIN
Rohingya activist Wai Wai Nu, who says her family has been in Myanmar for centuries, said census-takers at her Yangon home refused to list her as Rohingya, saying it was not permitted.
When she demanded written proof, she was told it was a verbal order.
The 27-year-old activist said the vast majority of Rohingyas insisted on being recorded by their ethnicity.
"Our ethnic identity is very important to us for getting equal rights with other people in Myanmar," she said.
Repression during nearly 50 years of military rule kept ethnic tensions in check in one of Asia's most diverse countries. But these have burst into the open since 2011, when a quasi-civilian government took power.
The country has endured several spasms of violence pitting Rakhine Buddhists against Rohingya. At least some of the attacks were blamed on Buddhist extremist groups.
Critics argue that Myanmar's government and the United Nations Population Fund (UNFPA) knew the census would be problematic before it began, but ignored the concerns.
Rights groups say the government is deliberately preventing the Rohingyas from being counted.
"The writing was on the wall and everyone knew it," said Matthew Smith, executive director of Fortify Rights, a rights group based in Southeast Asia.
"The government never had any intention of recognizing the Rohingya ethnicity through the census."
Trouble broke out last month when 400 rioters in Sittwe damaged offices, homes, warehouses, and vehicles belonging to aid groups and the U.N.
International aid workers withdrew.
Minister of Immigration and Population Khin Yi, who is head of the census commission, said the Rohingyas' classification was one of the reasons counting had to be extended, adding that some actually wanted to be counted as Bengalis.
"We heard from some people that they would like to be identified as 'Bengali', as they really are, but they are not brave enough to do so because of the pressure and threats behind them," the Democracy Today Daily quoted Khin Yi as saying, comments that a senior ministry official later confirmed.
"They were always recorded as Bengali, since the censuses under the British (colonial rule) till the last one in 1983."
The problem is not limited to the Rohingyas.
The government and UNFPA have been criticised for basing the census on 135 officially recognised ethnic groups. Critics say that is outdated and inaccurate.
Ethnic groups say their political representation and claims to ethnicity could be compromised if they are undercounted.
According to Human Rights Watch, several armed ethnic rebel groups said they would bar census-takers - enumerators - from their territory.
Questions have been raised about the validity of the census.
"If UNFPA and the government heeded warnings to at least remove the ethnic and religious questions, then a partial census would have been better than none at all," said Smith.
"At this point, it would've been better for the country if the enumerators stayed home." (Additional reporting by Soe Zeya Tun; Editing by Ron Popeski and Pravin Char)