Policemen on Wednesday arrested Arjun Narsingh K C, a senior leader of the Nepali Congress party that had planned peaceful protests on Friday to force the king to restore democracy.
Police arrested Arjun when he came to the party office in a Kathmandu suburb for the first time since the king clamped a state of emergency and banned protests, detained key party leaders and suspended fundamental rights.
The King justified his seizure of power on 1 February, saying that squabbling parties had failed to crush a violent Maoist revolt aimed at overthrowing the constitutional monarchy and establishing a communist republic.
"We are going to launch a movement, a peaceful movement, from 18 February and urge all political parties who are against the takeover to join," Arjun told reporters half an hour before being driven away in an iron-meshed blue police van.
King Gyanendra's takeover has
been widely condemned
Minutes later, Meena Pandey, chief of the party's women's wing, was hauled out by police and put into a pick-up van.
Political activists have attempted to organise small protests against the King's power grab, but heavy security in and around the temple-studded capital has foiled most protests.
Authorities have allowed rallies in support of the King.
Party officials said hundreds of people, including political activists, student leaders, trade union representatives and human rights workers, have been detained or put under house arrest across the country since the king's move.
Gyanendra's power grab, which has curtailed press freedom, has been condemned across the world, including by the United States, neighbouring India and Britain.
Washington, Paris and London recalled their ambassadors this week, and New Delhi has summoned its envoy from the world's only official Hindu nation for consultations.
Though some politicians have been freed, the chiefs of main political parties who are seen as capable of organising popular protests remain under house arrest or detention.
"We can't align ourselves with the Maoist violence"
Arjun Narsingh K C,
Nepali Congress Party
Arjun said his party would fight for the restoration of democracy but would not join hands with the Maoists.
"We can't align ourselves with the Maoist violence," Arjun said.
More than 11,000 people have been killed in impoverished Nepal in the revolt which began in 1996.
Maoist rebels, who hold sway over large parts of the country, have launched a nationwide transport strike since the weekend, crippling local trade to protest against the king's move.