Thiruvananthapuram in the southern state of Kerala implemented what it called a "triple lockdown" this week, as India overtook Russia to record the world's third-highest number of coronavirus infections.

Kerala's strict early measures to curb the coronavirus's spread meant it had just about 100 cases in May, a scenario that propelled its health minister - a retired teacher with a previously low profile - to rockstar status.

But since then nearly half a million people, mostly migrant workers, have arrived back in Kerala from abroad or from other Indian states. On Wednesday, the state recorded the highest single-day spike of 301 infections, taking the total to 6,301.

Kerala's Chief Minister Pinarayi Vijayan, who had voiced concerns about an outbreak if people were not tested before coming back, has attributed the rise in numbers to the returnees, saying they account for more than 80% of coronavirus cases.

"The city seems to be sitting on an active volcano," said Kadakampally Surendran, the state minister in charge of the area, urging people to "strictly follow" the lockdown measures.

Thiruvananthapuram's deputy mayor Rakhi Ravi Kumar said the rise in cases risked becoming unmanageable as people continued to return to the already densely populated city.

Residents are allowed to go out to buy essential items like groceries and medicine only between 7a.m. and 11a.m. Each neighborhood can have one shop open.

"People are allowed only to the nearest shop in the area. If they go further, the police will stop and send them back. We are implementing it strictly to prevent the virus from spreading further," she told the Thomson Reuters Foundation.

Lockdown rules

Neighborhoods have been sealed with a single entry and exit point under the "triple lockdown", which refers to movement of people and vehicles; households with infected people, and clusters where people who have had contact with infected people live.

People have been advised to stay at home and those who go out must carry signed forms declaring why for police inspection. All offices, businesses and public transport have been shut.

City inhabitant K. X. Thomas said there were police everywhere and he had been stopped three times when he went to buy fish in the morning.

"Police allowed me to pass only after ensuring that I had a mask and I was going to buy essentials. This is the strictest enforcement of lockdown I have seen," he said.

The state has made wearing masks and social distancing mandatory for a year, and banned spitting in public.

Violating these rules can result in a fine of up to 10,000 rupees ($133) or two years in prison.

Divya Gopinath, a city deputy commissioner of police, said nearly 220 people were arrested in the first two days of the new lockdown and 208 vehicles seized.

"We have been very strict in implementing the lockdown," she said.

"We deployed police personnel on all roads and streets ... and have given them instructions to arrest the violators and shift them to institutional quarantine centers."

Social media users in other states where cases have risen sharply, such as Tamil Nadu, Karnataka and Assam, urged authorities to implement similar lockdowns.

Several states have either already introduced weekend curfews and extended existing lockdowns as the situation worsens.

India has more than 740,000 cases, trailing the United States and Brazil, according to a tally by Johns Hopkins University.

"This is a right step to contain the spread of the virus," said Sreejith N. Kumar, former state president of the Indian Medical Association, of Kerala's lockdown. "It is heartening to note that people are cooperating with the authorities."