One protest leader said at least 300 people had been detained, in addition to another 100 arrested on Wednesday.
Khadga Prasad Oli, an opposition leader, said student leaders, women activists and party members were among those taken.
"We had planned demonstrations in 17 places," he said. "They were successful at 16, but at one point the police presence was so much that the protesters could not carry out their action."
The nationwide strike has brought much of Nepal to a standstill, with roads across the country deserted apart from a heavy security presence by the police and army.
Most businesses and schools were closed despite the government urging people not to support the strike.
The four-day strike is intended to step up pressure on King Gyanendra more than a year after he seized absolute power.
Earlier, Maoist guerrillas attacked Malangwa, a district town 350km southeast of the capital, Kathmandu, leaving at least 10 people dead.
The leftist rebels, fighting for the past decade to overthrow the monarchy, stormed through the town, firing at soldiers guarding government offices and security posts.
They attacked a jail and freed more than 100 inmates, among them some of their comrades, before fleeing.
"We will defy the ban on protests and take out rallies today from different places in the capital"
a leader of the Communist Party of Nepal
Five policemen and two guerrillas were killed, a local journalist said. But authorities said they could not confirm casualty figures.
The rebels also claimed to have brought down an army helicopter overnight near the town of Malangwa, killing eight of the 10 soldiers on board.
"Our liberation army has been able to shoot down the sophisticated night vision helicopter. This ... has taken our fighting to a new level," they said in statement.
Government officials said it was unclear what had brought the aircraft down.
The rebels had earlier said they supported Thursday's general strike, and have pledged to suspend all violence in Kathmandu during the event.
The ceasefire did not mention any other areas of the country.
Nepal's seven main political parties have vowed to defy a government ban on protests to launch what they say will be a decisive campaign to restore democracy.
About 300 activists were
arrested in Kathmandu
Nepal, which has been struggling to contain the Maoist revolt, sank into deeper turmoil after King Gyanendra sacked the government and took power in February 2005.
At the time he said Nepal's politicians had failed to quell quell the Maoist insurgency.one of the 10 poorest countries in the world, reliant on aid and tourism to stay afloat.
The revolt has killed more than 13,000 people and wrecked the economy, making Nepal
The king's seizure of power triggered widespread outrage, sparked anti-monarchy rallies and has since led to a loose alliance between the Maoists and a range of political groups opposed to the king.
Although pro-democracy protests have become a regular feature, this week's rallies are expected to be the biggest to date.