Police in the Nepalese capital Kathmandu have prevented a former MP from boarding a flight to Pakistan.
Navaraj Subedi, who is a leader of the pro-left Jana Morcha Nepal, is now in police custody.
Security personnel told Mr Subedi that they had orders not to let him leave the capital.
Jana Morcha Nepal is a member of a seven-party opposition alliance that is protesting against the royal takeover.
Mr Subedi was among several politicians and journalists travelling to Islamabad to attend the South Asian Parliamentary Forum being organised by South Asia Free Media Association (Safma), a Pakistan-based media group.
Nearly a dozen of them have cancelled their visit in protest against the move to detain Mr Subedi.
Opposition parties have also condemned the action and have alleged that the royal government is continuing to harass and detain opposition leaders and supporters even after the withdrawal of the state of emergency.
The government had imposed a nationwide state of emergency in February after King Gyanendra seized direct control of the government, which was withdrawn late last month.