BEIJING — Hundreds of Chinese police clashed with thousands of “underground” Christians over the demolition of a church that had been deemed an illegal structure.

Up to 500 police forced back as many as 3,000 Christians who had gathered during the weekend to mount a peaceful demonstration against the state’s demolition of the church in the eastern province of Zhejiang.

More than 20 Christians were injured in the clashes Saturday, while five organizers of the protest were arrested, according to the Hong Kong-based Information Center for Human Rights and Democracy.

Its spokesman, Frank Liu, said locals were rebuilding the unregistered church after it was destroyed last year by a typhoon.

As news spread that a demolition crew had been sent in, demonstrators massed at the construction site. Police were called in, leading to clashes.

A police officer involved in the incident acknowledged that the clash took place but denied that any Christians had been arrested.

The unnamed officer said: “It is clear that this church was an illegal structure. It did not have the approval of the Religious Affairs Bureau or the government. The church had to be destroyed. They can explain their demands, but if they are in violation of public order, laws, and regulations, we will deal with them in accordance with the law.”

China maintains tight control over religious affairs and requires all faith communities, whether Buddhist, Taoist, Islamic, or Christian, to register and carry out their activities according to strict and ever-changing guidelines.

There are an estimated 80 million Christians praying outside China’s state-regulated church. At Christmas and Easter, elevators and stairwells of public housing complexes are packed with smartly dressed Christians on their way to a service at a “house church.”

Last year, President Bush attended one of Beijing’s five officially recognized Protestant churches during a visit to the country. He wrote in the visitors’ book: “God bless China’s Christians.”

A spokesman for Amnesty International in Hong Kong, Mark Allison, said: “This latest incident proves that despite the odd overture towards more freedom of expression and the building of a harmonious society, China’s leaders will use force to crack down on unsanctioned religion.

“Those arrested, like the thousands of Christians interned across the country, who largely go unreported, are forbidden to worship the religion of their choice. They will receive summary justice by the police and be sentenced to re-education through forced labor or sentenced to prison,” he said.

While Beijing has been doing battle with the Vatican for decades and appointing its own priests, it is also struggling to rein in new and growing religious movements that have attracted millions of new followers in recent years.

The most prominent target has been the Falun Gong spiritual movement, which was banned in 1999 as a threat to public safety and communist rule. Its worshippers are regularly tortured.