King Gyanendra seized absolute power in the Himalayan nation on 1 February 2005, saying the move was needed to quell a decade-old revolt by Maoists battling to overthrow the monarchy and install a communist state.
But the takeover, which led to arrests of political activists and suspension of civil liberties, sparked international condemnation and triggered wide protests in the tiny nation wedged between India and China.
Krishna Prasad Sitaula, a Nepali Congress Spokesman, said: "Marking the one-year anniversary of the royal takeover, the seven-party (opposition) alliance is going to observe 1 February as a 'Black Day' across the country with protests."
Sitaula said: "The opposition alliance will rally from different parts of the Kathmandu Valley and converge on Durbar Square in the centre of the Capital."
In Nepal, waving black flags or wearing a black armband or headband is a way of registering protest.
King Gyanendra seized absolute
power on 1 February 2005
The announcement of the "Black Day" followed a one-day general strike called by the seven-party alliance on Thursday that paralysed the nation, shutting down offices and shops along with businesses and schools.
The parties called the strike to demonstrate their anger at a renewed crackdown by the king in which mobile phones were cut, a ban was imposed on protests and hundreds of activists were rounded up. Many are still in detention.
King Gyanendra, as part of a promise to restore democracy, has announced municipal polls for 8 February followed by national elections before April 2007.
But opposition parties have branded the local polls a sham directed at placating international criticism of the king's takeover.
Nepal's Maoists have vowed to disrupt the elections and have called for a week-long national general strike to coincide with polling day.
Activists of the seven opposition parties will fan out to the 58 municipalities across Nepal this weekend and organize protests in an attempt to persuade people not to vote in the polls, Sitaula said.