* About 50 killed on Sunday, most in Yangon
* Unidentified attackers torch Chinese factories
* China embassy urges Myanmar to stop violence, ensure safety
Myanmar security forces fired on pro-democracy demonstrators on Monday, witnesses said, a day after dozens of protesters were killed and attackers torched several Chinese-financed factories in the city of Yangon.
Supporters of detained democtratic leader Aung San Suu Kyi marched again including in Mandalay and the central town of Myingyan where police opened fire, witnesses said.
"They fired on us," an 18-year-old protester in Myingyan said. "One girl got shot in the head and a boy got shot in the face ... I've heard they died."
The protesters took to the streets in defiance of the authorities' escalating use of violence, with dozens killed on Sunday in the bloodiest day since the Feb. 1 coup.
The arson attacks on Sunday provoked China's strongest comments yet on the turmoil gripping its Southeast Asian neighbour, where many people see China as supportive of the Feb. 1 coup.
The Chinese embassy said many Chinese staff were injured and trapped in the attacks, and urged Myanmar's ruling generals to stop violence and ensure the safety of people and property.
Japan, which has long competed for influence in Myanmar with China, said it was monitoring the situation and considering how to respond in terms of economic cooperation.
The worst of Sunday's bloodshed came in the Yangon suburb of Hlaingthaya where security forces killed at least 37 protesters after arson attacks on Chinese-owned factories, said a doctor in the area who declined to be identified.
Sixteen people were killed in other places, rights group Assistance Association for Political Prisoners (AAPP) said, as well as one policeman.
Media said martial law had been imposed in Hlaingthaya and several other districts of Yangon, and in several parts of the second city of Mandalay.
The latest deaths bring the toll from the protests to about 140, based on a tally by the AAPP and the latest reports.
A junta spokesman did not answer calls requesting comment.
In an apparent bid to suppress news of the turmoil, telecoms service providers were ordered to block all mobile data nationwide, two sources with knowledge of the matter said. Telecom Telenor said in a statement "mobile internet was unavailable".
The army said it took power after its accusations of fraud in a Nov. 8 election won by Suu Kyi's party were rejected by the electoral commission. It has promised to hold a new election, but has not set a date.
Suu Kyi has been detained since the coup and is due to return to court on Monday. She faces at least four charges, including the illegal use of walkie-talkie radios and infringing coronavirus protocols.
The Hlaingthaya industrial suburb is home to migrants from across Myanmar. On Sunday, security forces opened fire as black smoke billowed from factories.
Army-run Myawadday television said security forces acted after four garment factories and a fertiliser plant were set ablaze and about 2,000 people had stopped fire engines from reaching them.
Violence also broke out in other parts of Yangon as protests ran late into Sunday night with several people killed, residents said.
Western countries have condemned the violence and Asian neighbours have offered to help resolve the crisis but Myanmar has a long record of rejecting outside intervention.
Tom Andrews, the United Nations human rights investigator on Myanmar, appealed for U.N. member states to cut the supply of cash and weapons to the military.
"Heartbroken/outraged at news of the largest number of protesters murdered by Myanmar security forces in a single day. Junta leaders don't belong in power, they belong behind bars," he said on Twitter.
China's embassy described the situation as "very severe" after the attacks on the Chinese-financed factories and it urged authorities to "stop all acts of violence, punish the perpetrators in accordance with the law and ensure the safety of life and property of Chinese companies and personnel".
Anti-Chinese sentiment has risen since the coup, with opponents of the army takeover noting Beijing's muted criticism compared with Western condemnation.
China's Global Times newspaper blamed instigators for the arson and called for their punishment. It said China was trying to promote a peaceful settlement of the crisis.
Protest leader Thinzar Shunlei Yi said Myanmar people did not hate their Chinese neighbours but China's rulers had to understand the outrage felt in Myanmar over their stand.
"Chinese government must stop supporting coup council if they actually care about Sino-Myanmar relations and to protect their businesses," she said on Twitter.
A senior official from Suu Kyi's National League for Democracy party, Mahn Win Khaing Than, who is on the run along with most senior party officials, said on the weekend the civilian government would give people the legal right to defend themselves. It announced a law to that effect on Sunday.
People transport a person who was shot during a security force crackdown on anti-coup protesters yesterday in Thingangyun, Yangon, Myanmar