JAKARTA, June 4, Reuter - The leader of Indonesia's free trade union Solidarity was seized by plainclothes men carrying guns on Sunday and has not been heard of since, union activists said on Tuesday.

Military, police and security officials say they know nothing of Saut Aritongan's whereabouts, but a military intelligence source said privately that the activist was being held by the regional military command and had not been physically hurt.

The apparent arrest was badly timed, analysts said.

Senior economic minister Radius Prawiro flew to the Netherlands on Tuesday to lobby for aid from the Inter Governmental Group on Indonesia (IGGI), a group of donor nations and agencies expected to lend Jakarta 4.6 billion dollars this year.

IGGI chairman Johannes Pronk said during a visit to Indonesia last month that Western governments must speak out about labour abuse in Indonesia, press Jakarta to recognise Solidarity and allow other independent unions.

Jakarta, which sells itself to foreign investors as a source of cheap labour, says the single government-backed union SPSI is all the country needs and has refused to recognise Solidarity.

Saut, Solidarity's secretary general, was stopped in broad daylight on a major Jakarta thoroughfare on Sunday afternoon by five men with guns who cut off the taxi he was in and forced him into their car, activists said.

A Solidarity member who was with Saut said the men had no arrest warrants and did not say which security, intelligence or other agency they were from.

The Coordinating Minister for Politics and Security, Sudomo, a former manpower minister who has little time for Solidarity, told Reuters he knew nothing of Saut's arrest, as did military spokesman Nurhadi Purwosaputro.

"But I just can't understand why he would have been arrested (by military or intelligence officers). We have no business with him," Nurhadi said.

Officials at the police and the Attorney General's office said they had no record of a warrant for Saut's arrest.

The activist was picked up ahead of a Solidarity-organised demonstration on Monday morning by about 300 workers outside the offices of the International Labour Organisation (ILO).

Solidarity objects to Indonesia's attempts, likely to succeed, to chair the next session of the ILO's annual conference. The union says Jakarta violates the constitutional rights of Indonesian workers by not allowing them to organize.

Some labour analysts said the Indonesian government would if necessary put security ahead of worker welfare regardless of world opinion.

"They would rather show they are heavy-handed than out of control. They're worried about investors who really prefer to see these guys kept down," said one foreign labour analyst.

Indonesia's minimum wage of around 60 U.S. cents a day, widely abused according to Manpower Minister Cosmas Batubara, is a magnet for investors from countries like Japan, Taiwan and South Korea whose own labour costs are spiralling upwards.