Officials say they detained political leaders and activists in the early hours Thursday. They also cut telephone and Internet communications for several hours, before restoring all but the cell phone networks later in the day.
The moves are an apparent attempt to prevent what opposition leaders say could be tens of thousands of people taking to the streets of the capital Friday for a demonstration against the government of King Gyanendra.
The massive rally comes weeks before the first anniversary of the king's dismissal of parliament last February 1. He said he was forced to act because Nepal's political parties had failed to stop a long-running insurgency by communist rebels.
To carry out his takeover, King Gyanendra arrested opposition leaders, temporarily cut phone lines, and censored the media.
Rajan Bhattarai is a member of the Communist Party-United Marxist and Leninist, one of the largest parties in the coalition government the king dismissed last year.
He says the government's actions to stop the demonstration are a sign that the king has begun to panic about his failure to fulfill his promises.
"What he promised to the people is peace, what he promised to the people when he took over is democracy," said Bhattarai. "And if you look at the past one year of his rule, there is no peace, no democracy."
Friday's planned demonstration also is meant to protest the king's plans to hold local elections next month, which he calls a step toward restoring democracy. King Gyanendra has promised that national parliamentary elections will be held next year.
Mr. Bhattarai calls the elections a farce.
"He wants to use it to hoodwink the international community, that OK, I have done this, I am in the process of [restoring] democracy. But in fact, in practice, it is not," he concluded.
Earlier this week, the government imposed a curfew on Nepal's cities and banned rallies.
Authorities said they fear the rallies could be infiltrated by communist insurgents, who might use terror tactics to disrupt them.
The insurgents have waged a 10-year campaign to topple the monarchy and have formed a loose alliance with a coalition of political parties to oppose the planned elections.