Police are seen at a road block in Christchurch, New Zealand, March 15, 2019. [Photo/Xinhua]

The terrorist attack in Christchurch has left 40 dead and over 20 seriously injured on Friday, New Zealand Prime Minister Jacinda Ardern said.

Four suspects were arrested, including an Australian-born man. At this stage the government had no reason to believe there are any other suspects, she added.

"It is clear that this can now only be described as a terrorist attack," Ardern told a press conference in Wellington, adding that the attack appeared to have been "well-planned."

Two explosive devices attached to cars have been decommissioned by police.

It was announced that the national security level in New Zealand was raised from low to high.

At least two gunmen opened fire at two separate mosques in Christchurch on Friday.

According to reports, a gunman in camouflage uniforms opened fire inside the Masjid Al Noor mosque full of prayers in central Christchurch during the afternoon praying time.

It was understood that gunshots happened at least in six different places.

Witnesses described that there were much blood and multiple deaths. At least 50 rounds of gunshots were heard during the shooting, said one witness.

New Zealand Police Minister Mike Bush confirmed that three men and one woman were in police custody in relation to the multiple shootings at two separate mosques in central Christchurch.

Bush said he was aware that footage of the Al Noor Mosque shooting was on social media and police were doing everything they could to get it removed.

"This is one of New Zealand's darkest days. My thoughts are with Christchurch," Ardern said in her earlier address.

"It is an unprecedented violence in New Zealand. There is no place for such extreme violence in New Zealand," she said.

People were recommended by the police to stay inside. All mosques nationally were urged to shut their doors, and people were advised to refrain from visiting these premises until further notice.

Local streets and schools were still locked down.

Christchurch airport was operating as normal.

Chinese Consul General in Christchurch Wang Zhijian told Xinhua that emergency response has been activated. No casualty of Chinese citizen has been reported yet.

The Chinese Consulate General office is working with local police to check if there is any Chinese national being caught in the gunfire.