* U.N. Commissioner voices concern on number of arrests
* Says situation could worsen if militia act outside law
By Stephanie Nebehay
GENEVA, June 19 (Reuters) - U.N. High Commissioner for Human Rights Navi Pillay called on Iran on Friday to rein in Islamic militia accused of violence against protesters and warned that the situation in the country could still deteriorate.
Pillay also voiced concern about the growing number of human rights activists and opposition members arrested since the presidential election a week ago and urged authorities to uphold due process of law.
"It is the responsibility of the government to ensure that militia members and regular law enforcement agencies do not resort to illegal acts of violence," she said in a statement.
"If they are perceived to be acting outside the law, it could provoke a serious deterioration in the security situation, which would be a great tragedy and is in nobody’s interests."
Her appeal came a day after an unannounced meeting with Iranian Nobel Peace Prize laureate Shirin Ebadi. It also coincided with Supreme Leader Ayatollah Ali Khamenei appealing for calm in Iran after days of street protests against the results of the election, won by incumbent Mahmoud Ahmadinejad.
Pillay, a former United Nations war crimes judge, said "possible illegal use of excessive force and acts of violence" by some members of the Basij militia may violate both domestic Iranian and international law.
Basij is a volunteer paramilitary force fiercely loyal to Khamenei, who has the final say on all matters of state in Iran. During the past days of violence, police have accused "bandits" of setting buses on fire, breaking windows of banks and other buildings and damaging public property.
Iranian state media have reported seven or eight people killed in protests since the election.
Pillay praised the "largely peaceful and dignified conduct of the huge demonstrations that have been taking place" in Tehran. The legal basis for the arrests was not clear and the whereabouts of some detainees were not known, she said.
"These are all troubling questions and I urge the Iranian authorities to ensure that due process is followed, since to do otherwise may fan the feelings of injustice," she said.
Her spokesman Rupert Colville said the number of arrests "would appear to be in the hundreds".
Ebadi, in an interview with Reuters, called for the disputed election results to be annulled and for fresh elections to be held under the supervision of United Nations observers. The Iranian human rights lawyer also called for the unconditional release of about 500 people whom she said had been arrested in the past week, including two colleagues.
"The response of the government has been a very violent one," Ebadi said. "I asked Mrs Pillay to do her utmost to stop and prevent this violence that is being done against defenceless people, against people who are protesting in a peaceful manner."
(Editing by Mark Trevelyan)