Visibility was reduced to 300 metres at 9.30 am as thick smog created by smoke and fog descended in the wee hours of the day, the Met Department said.

The visibility later improved to 1,000 metres at around two pm. The smog continued for long as there was no wind, they said.

Seven students were injured when a school bus was hit by a truck at around 7.30 am near Kashmere Gate area near the walled city.

Students of DPS Mathura Road were getting into their mini-bus near when the truck hit the vehicle from behind. Three girls and four boys sustained injuries as the vehicle overturned, police said.

They were immediately rushed to Bara Hindu Rao Hospital. While six of them were discharged after treatment, one student still remains admitted. The bus driver was also injured in the incident.

The truck driver fled the spot after the incident. In a separate accident, five railwaymen, including an officer, were run over by Delhi-bound Dehradun Janshatabdi Express near Ghaziabad at around 11.30 am.

Poor visibility was believed to be the cause of the mishap.

Northern Railway sources said a railway inspection officer and four gangmen were killed in the accident between Ghaziabad and Shahibabad stations. They said the mishap took place when the gangmen were at work.

Smog is experienced at the onset of winter, the Met office said. Similar conditions may prevail for the next few days, it said.