At least 38 people have been killed after a packed passenger bus ran over a suspected rebel landmine in southern Nepal, the army says.
The blast ripped the bus apart
Another 70 were hurt, many seriously, in the blast in Chitwan district. State radio put the number killed at over 50.
The vehicle and many of those crammed inside and on top were blown to pieces.
The rebels have yet to comment on the blast, the single bloodiest incident involving civilians since they began their fight for a republic in 1996.
They have carried out landmine attacks in the past against security targets but do not usually target civilians in this way.
Nepal's government has called the incident a terrorist act.
'Littered with blood'
The bus, travelling from the village of Madi to nearby Narayangadh, was packed with villagers going to market or to work when it hit the landmine hidden under a bridge over the Bandarmudhe river.
The landmine appears to have been detonated from 200 metres away by a wire, tearing the bus apart and creating utter carnage.
Authorities say most of those killed died on the spot.
"The place is littered with blood... Many women and children have been killed," an army officer told Reuters news agency from the area.
The scene of the disaster is very close to the country's most famous national park, about 180km (110 miles) from the capital, Kathmandu.
Those injured have been moved to the district capital or to Kathmandu by ambulance and helicopter.
Survivors spoke of bodies and body parts flung far and wide.
"Our bus was thrown in the air. The bus was ripped into pieces and many people were killed," one survivor, Khum Bahadur Gurung, 62, told the Associated Press from hospital.
Reports said local hospitals were struggling to cope with the numbers needing treatment.
Families of the victims flocked from villages to the town of Chitwan for news of the casualties.
An army spokesman in the capital told the BBC that the type of explosive device showed the incident was definitely the work of what he called Maoist terrorists.
He said he did not know why the Maoists would target civilians in this way.
The BBC's Charles Haviland in Kathmandu says the rebels have tended to avoid indiscriminate attacks on civilians, instead ruthlessly targeting those they decide are their enemies.
However, they have in the past said that they regard buses carrying military personnel as legitimate targets, even in cases where most of the passengers are civilians.
The army says that in the Chitwan incident some of the passengers, including some who died, were off-duty soldiers.
Nepal's army has been unable to beat the rebels
Our correspondent says if this is the work of the Maoists as seems likely, it will do no good at all to their attempts to woo Nepal's political parties, which shun violence.
Major opposition parties were swift to blame the rebels for the attack, calling it inhuman and barbaric.
About 12,000 people, many of them civilians, have died in Nepal's conflict, according to the military's figures.
Human rights groups accuse both rebels and the security forces of abuses.
Violence has escalated since King Gyanendra seized direct power in February, saying the government had not done enough to end the insurgency.
Nepal is heading towards a civil war. The concentration of power on the hands of Gyanendra will surely increase the recruiting for the Maoist rebels. The only way to combat the rebels is increasing the conditions of the population in such a poor country as Nepal. It's sad, but the more violence the conflict becomes, less tourists will visit the country and worse will be the perspectives for the population. It is a snowball.
Breno, Brasilia, Brazil
Having also just returned from my second visit to this beautiful country I am saddened at this news. The ideals the Maoist terrorists cannot and will not ever be achieved. Okay it worked for Mao in 1949 in China but the world has changed (and so has China). Communism with its ideals has no place in today's world and especially in the Kingdom of Nepal.
Sue Robb, London, UK
Incredibly shocking! What is the King doing? If he cannot control the nation and maintain law and order, then what is the difference between him and the political parties? Such act of brutality must be condemned by the whole world.
Abhinav Pathak, Logan, Utah, USA
It's one of the saddest incidents by Maoists in last 12 years. It will surely deduct the trust from Nepalese people who were supporting them. Killing all those innocent people can never bring a change that they are expecting. Maoists have really proved themselves as terrorists from this incident.
Raj Silwal, Gent, Belgium
After living in that beautiful country for six months it saddens me to hear such news. It angers me even more that the Maoists rebels claim to be for the people. They destroy any claims of being for the Nepali people when they consistently hurt and murder.
Jessica Leslie, Vancouver, BC, Canada
This is the time for all the political parties to give away their political prejudices and fight against the terrorism. Let's go to protect the lives of the citizen in the villages than chanting the slogans at the street in the capital which has nothing to do for the pains of the citizen outside the valley.
PM Bhandari, Seoul, Korea
Nepal is endowed with the most stunning natural beauty and among the most generous and kind people in the world. Violence like this does nothing but bring harm to the reputation of the country. No doubt everyone in Nepal has become all too accustomed to these types of reports. It is tragic that many Nepalese must suffer death and that the country must suffer depression because of the terrible actions of a few radicals.
Andy Hiller, New York, United States
Whoever did this is very cowardly, inhumane and a terrorist. The Royal Administrations, Maoists as well as all Politicians should take the responsibilities for happenings such as this brutal accident in such a peaceful country. The world should not be blind. Please, do not let them kill innocent people anymore.
Loken B, Dhangadhi, Kailali, Nepal
There are no words to condemn such an attack to the unarmed civilians. This might be the bloodiest attack ever in Nepal. This is the ruthless, barbaric, brutal, merciless, cruel sin to the peace loving Nepalese.
R R Adhikari, Germany
This is a one of the most brutal incidents by rebels in the last 12 years after insurgency. All of us peace seekers think this is a negative point for peace talks between Maoists and the government. I'm from the area but am not a witness. It is really brutal and inhumane too.
Rohini Acharya, Narayangarh, Chitwan
Such an inhuman incident, definitely breaks the relation of the parties, civil society and human rights associations. This single incident takes the hope of peace far away. Such activities give moral power to the throne that is ruling the country. In a single word, such brutality is condemnable.
Jayram Gautam, Kathmandu
This represents another sad episode in an escalating conflict. The uprising is largely unwanted, largely unsupported and wholly crippling to the weak economy of Nepal. After recently returning from this beautiful country, I find it deeply distressing that the quiet, proud people of this kingdom continue to suffer loss of life, earnings and hope.
Simon Nurse, Cardiff, Wales
Such news would have made a big deal in most other countries, but here this news would probably be forgotten in a few days and people move on with their lives as if nothing has happened. The killings have become such a routine thing over here, with a daily body count announced at the end of the day in the government news, most of which goes to deaf ears.
Kamal Nepal, Kathmandu, Nepal
I have just recently returned from three months in Nepal and also from having visited the Chitwan district. It is a real setback as it was already quite evident that people were staying away from this area of the country. An incident like this will only reconfirm the fears that a lot of visitors already have of the country. If it wasn't hard for Nepal before then it certainly is now.
Edward White, London, England
All the members of the Cabinet and King Gyanendra should give their resignations.
Bird, Kathmandu, Nepal
Unfortunately, the fear is that daily lives lost here are being counted today and the very same tomorrow wakes up forgetting them. Hopes for peace are way too distant and this incident yet again pushes that even further. But sooner or later, it will make a difference, it has to. Those who are responsible for all these should prepare for accounting one day.
Sandesh Acharya, Kohalpur, Nepal