YANGON (REUTERS) - An official from the party of detained Myanmar leader Aung San Suu Kyi died overnight in police custody, associates said, while security forces cracked down on demonstrators staging some of the most widespread protests against the Feb 1 coup.

The cause of Khin Maung Latt's death was not known, but Reuters saw a photograph of his body with a bloodstained cloth around the head.

Sithu Maung, a member of the dissolved Parliament, said in a Facebook post that Khin Maung Latt was his campaign manager and was arrested on Saturday night in the Pabedan district of Yangon.

Police in Pabedan declined to comment.

In Mandalay, police fired stun grenades and tear gas to break up a sit-in protest by tens of thousands of people, Now media group said. At least 70 people were arrested.

Earlier, troops occupied a university in the city after firing rubber bullets at people there, it said. Two people were injured.

Police also launched tear gas and stun grenades in the direction of protesters in Yangon and in the town of Lashio in the northern Shan region, videos posted on Facebook showed.

A witness said police opened fire to break up a protest in the historic temple town of Bagan, and several residents said in social media posts that live bullets were used.

Video posted by media group Myanmar Now showed soldiers beating up men in Yangon, where at least three protests were held despite overnight raids by security forces on campaign leaders and opposition activists.

China's stance

On Sunday, Chinese top diplomat Wang Yi said that China is willing to engage with all parties to ease the crisis in Myanmar and that it is not taking sides.

Beijing has said the situation in Myanmar, where the military seized power last month, was "absolutely not what China wants to see" and has dismissed social media rumours of Chinese involvement in the coup as nonsense.

"China is... willing to contact and communicate with all parties on the basis of respecting Myanmar's sovereignty and the will of the people, so as to play a constructive role in easing tensions," Mr Wang told a news conference on the sidelines of China's annual gathering of Parliament.

While Western countries have strongly condemned the Feb 1 coup, China has been more cautious, emphasising the importance of stability.

Still, China has agreed to a United Nations Security Council statement that called for the release of civilian government leader Suu Kyi and other detainees, and voiced concern over the state of emergency.

"China has long-term friendly exchanges with all parties and factions in Myanmar, including the National League for Democracy (NLD), and friendship with China has always been the consensus of all sectors in Myanmar," Mr Wang said.

"No matter how the situation in Myanmar changes, China's determination to promote China- Myanmar relations will not waver, and China's direction of promoting China-Myanmar friendly cooperation will not change," Mr Wang said.

Myanmar junta's lobbyist

Israeli-Canadian lobbyist Ari Ben-Menashe, hired by Myanmar's junta, told Reuters the generals are keen to leave politics and seek to improve relations with the United States and distance themselves from China.

He said Suu Kyi had grown too close to China for the generals' liking.

Ben-Menashe said he had also been tasked with seeking Arab support for a plan to repatriate Muslim Rohingya refugees, hundreds of thousands of whom were driven from Myanmar in 2017 in an army crackdown after rebel attacks.

Over 50 killed

The United Nations says security forces have killed more than 50 people to stamp out the daily demonstrations and strikes in Myanmar since the military overthrew and detained Suu Kyi on Feb 1.

"They are killing people just like killing birds and chickens," one protest leader said to the crowd in Dawei, in the country's south. "What will we do if we don't revolt against them? We must revolt."

Yangon residents said soldiers and police moved into several districts overnight, firing shots. They arrested at least three people in Kyauktada Township, residents there said. They did not know the reason for the arrests.

"They are asking to take out my father and brother. Is no one going to help us? Don't you even touch my father and brother. Take us too if you want to take them," one woman screamed as two of them, an actor and his son, were led off.

Soldiers also came looking for a lawyer who worked for Ms Suu Kyi's NLD but were unable to find him, said Sithu Maung in a Facebook post.

'Punched and kicked'

Well over 1,700 people had been detained under the junta by Saturday, according to figures from the Assistance Association for Political Prisoners (AAPP) advocacy group. It did not give a figure for overnight detentions.

"Detainees were punched and kicked with military boots, beaten with police batons, and then dragged into police vehicles," AAPP said in a statement.

"Security forces entered residential areas and tried to arrest further protesters, and shot at the homes, destroying many," the statement said.

Myanmar authorities said on Saturday they had exhumed the body of 19-year-old Kyal Sin, who has become an icon of the protest movement after she was shot dead in Mandalay last Wednesday wearing a T-shirt that read "Everything will be OK".

State-run MRTV said a surgical investigation showed she could not have been killed by police because the wrong sort of projectile was found in her head and she had been shot from behind, whereas police were in front.

Photographs on the day showed her head turned away from security forces moments before she was killed. Opponents of the coup accused authorities of an attempted cover-up.

The killings have drawn anger in the West and have been condemned by most democracies in Asia. The United States and some other Western countries have imposed limited sanctions on the junta.

Protesters demand the release of Ms Suu Kyi and the respect of November's election - which her party won in landslide but which the army rejected

The army has said it will hold democratic elections at an unspecified date.

Junta leader and army chief Min Aung Hlaing had been under Western sanctions even before the coup for alleged human rights abuses against the Rohingya and other minorities.