An influential Protestant church in China says prominent leaders were arrested in what appears to be a growing crackdown on the underground church movement.

Nine people were detained on Tuesday after police raided their homes and the church office in Chengdu, the Early Rain Covenant Church said. Five of them had been released by Wednesday.

More than 1,000 miles away in Wenzhou, authorities began demolishing the Yayang Church building, as seen in video obtained by the non-profit ChinaAid, which monitors religious persecution.

This latest wave of arrests, after others last year, shows the Communist Party's resolve to snuff out churches that do not align with its ideology, Christian groups say.

The BBC has contacted China's embassy in the UK for comment. Authorities have not made any statements about the arrests, or the demolition in Wenzhou.

China promotes atheism and controls religion. The government said in 2018 there are 44 million Christians in the country, but it's unclear if that number includes those who attend the many underground churches.

The Party has long pressured Christians to join only state-sanctioned churches led by government-approved pastors.

But Christian groups say the grip has tightened noticeably, with arrests becoming more common and prompt. At least two church leaders in China have told the BBC that authorities are swiftly arresting unauthorised church leaders, while in the past, these individuals would first be warned, then fined and finally detained if they still refuse to comply with orders.

Just weeks ago, Li Yingqiang, the current leader of Early Rain Covenant Church, had said he "sensed a storm gathering" and referred to "the imminent prospect of... another large-scale crackdown".

"I dearly hope that none of our families shall ever again endure such a storm. Yet as an elder appointed by the Lord to stand among you... it is my duty to remind you all to prepare yourselves before the storm returns," he wrote in a letter to church members in November.

Li and his wife Zhang Xinyue are among the four who remain in detention. Their church described the arrests as a "concerted operation" but said the grounds for arrest, and whether those detained have been charged, remain unclear. It added that it had lost contact with two other members but did not say they had been detained.

"The situation is ongoing, with specific details yet to be fully confirmed," Early Rain Covenant Church (ERCC) said in a statement to members and supporters. It also also sought prayers for its members' safety and their perseverance in the Christian faith.

In Wenzhou, local authorities brought in bulldozers, cranes and heavy machinery earlier this week, and started taking down part of the Yayang Church building, as seen in the video. ChinaAid said it was told by multiple sources that hundreds of armed and special police officers have been deployed to stand guard outside the building.

Sometimes known as "the Jerusalem of China", Wenzhou has more Christians than any other city in the country. Residents living near Yayang Church have been "driven away", while those working in the area has been instructed not to take photographs or record video, ChinaAid said.

"The massive mobilisation against the two major independent church networks shows the central government is determined to stamp out Christian churches entirely, unless the church is totally indoctrinated into the party's ideology," said Bob Fu, who founded ChinaAid.

In December, authorities arrested about 100 members of Yayang Church in Wenzhou over five days. At least 24 members remain in custody, according to Human Rights Watch. And in October last year 30 leaders of Zion Church, one of China's biggest underground churches, were rounded up across seven cities. Founder Ezra Jin is still in custody.

The Chinese government has also targeted the Early Rain Covenant Church, founded in 2008, for years. In 2018, authorities raided the church and arrested founding pastor Wang Yi and his wife Jiang Rong. At least 100 church members were taken into custody in the following days in one of China's largest crackdowns on churches in the past decade.

Wang, an outspoken critic of the Communist Party's policies on religion, was jailed for "inciting subversion of state power" and "illegal business operations". He is due to be released in 2027.

The church has continued to gather online and sometimes replays Wang's recorded sermons to its members.

"Xi Jinping's government has tightened ideological control and intensified its intolerance of loyalties beyond the Chinese Communist Party," said Yalkun Uluyol, China researcher at Human Rights Watch.

"Concerned governments and religious leaders around the world should press the Chinese government to free detained religious adherents and respect religious freedom in China," Uluyol added.

Under Xi, China has increased its control on religious freedom. Since 2015, he has called for the "Sinicisation of religions" which requires religious doctrines and practices to conform with Chinese culture and values.

Last year, authorities banned clergy of all religions from preaching live on social media, organising online activities for children, and raising funds online, unless these are carried out on government-approved platforms.