An Amnesty statement said: "More than 1350 people have been arrested since 19 January for either organising or participating in political demonstrations.
"Of those, at least 888 are believed still to be detained."
About 50 people, including 30 journalists, were detained in protests in Kathmandu on Wednesday but a demonstration against the king's rule fell flat after police moved in to prevent people gathering.
The journalists were released later on Wednesday, but many political leaders remain under house arrest, local activists say.
Kanak Mani Dixit, editor of Himal SouthAsian magazine, who was held briefly on Wednesday, said: "You did not see a mass mobilisation because the senior leaders who could mobilise people are in jail and have been for the last 10 to 12 days.
"But there were spontaneous protests nationwide."
Gyanendra sacked the elected government on 1 February 2005, saying it had not done enough to quell a Maoist insurgency, had failed to hold elections and was responsible for widespread corruption.
On Wednesday, the king marked the first anniversary of his takeover with a promise to push ahead with controversial local elections as part of his "road map" to democracy.
The main political parties are boycotting the elections and the Maoists have said they will take action against those participating.
King Gyanendra is moving ahead
with local elections
The rebels, who mark the 10th anniversary of their uprising on 13 February, have called a week-long nationwide strike against the elections starting on Sunday.
Since ending a unilateral ceasefire at the beginning of January, the Maoists have increased attacks on government targets, including a large attack in the western district of Palpa on Tuesday that killed at least 15 people.
The defence ministry said 4000 Maoists attacked at least five targets in Palpa district 400km (250 miles) west of Kathmandu on Tuesday night. It said that four rebels, four soldiers and seven policemen died.
The Maoists claimed that many more police and troops had been killed and threatened further attacks.
More than 12,000 people have died since the insurgency began.