The International Labour Organisation negotiated with the Fiji government to have a prominent trade unionist released from police custody last week.
Felix Anthony was let go without charge after two nights in police cells amid planning for worker protests.
After Mr Anthony's arrest last Wednesday, the ILO said it took immediate action to negotiate with the government - in coordination with the rest of the UN system - for his release and for the release of other trade unionists taken in earlier.
Mr Anthony, National Secretary of the Fiji Trades Union Congress (FTUC), was detained just after a meeting with the government and employers, part of tripartite discussions under the watch of the ILO to resolve several labour issues.
"The ILO is concerned about the arrest of Mr Anthony and stresses that it is the responsibility of Government to ensure that the International Labour Conventions on Freedom of Association that it has freely ratified are applied and respected by all state authorities in Fiji," the ILO said in answer to questions put by RNZ Pacific.
The UN agency said such precipitate actions do not help create the necessary constructive atmosphere for the tripartite talks.
Talks over the previous two weeks had been "cooperative and productive", the ILO said.
"We will continue supporting the trade unions and the other two parties with the aim of encouraging them to return to tripartite dialogue," the ILO said.