They were rushed to hospital, and most were discharged a few hours later.

Delhi Police has registered a case against "unknown persons" in the matter.

Students of Rani Jhansi Sarvodaya Kanya Vidyalaya and Government Girls Senior Secondary School, Number 2, in Tughlaqabad area, were entering class after the morning assembly session at around 7.30 a.m. when they felt dizzy with the toxic fumes. Students complained of eye and throat irritation. Around 30 students also fainted.

Their schools are adjacent to the Tughlaqabad container depot.

"A total of 475 students from Classes 6-12 from both the schools were affected by the fumes. They were treated at different hospitals," Deputy Commissioner of Police Romil Baaniya told IANS.

He said the chemical leaked from four drums containing chloro (methyl) phenylsilane, which was imported from China, and was enroute to Sonepat in Haryana. Each drum contained 220 litres of the chemical, which is used in the manufacture of pesticides.

The container, which reached Sonepat later, was being handled by a team of the State Disaster Response Force and a Delhi Police team, Baaniya said.

The Delhi government has launched a magisterial probe into the matter.

Baaniya said that police has registered a case against "unknown persons" under various sections of the IPC (Indian Penal Code) relating to making the atmosphere noxious to health, endangering life or personal safety of others, causing hurt by act.

"We are investigating to see how the chemical leaked," Baaniya said.

The affected students were rushed to four nearby hospitals -- Batra Hospital, Apollo Hospital, Majeedia Hospital and ESI Hospital.

"After treatment 406 students were discharged while the rest will be discharged soon," Baaniya said, adding that 12 students were referred to Safdarjung hospital, who were later discharged.

Delhi Fire Service said it received a distress call about the gas leak at 7.43 a.m. and rushed three fire tenders to the spot.

Seventeen ambulances from the Centralised Accident and Trauma Services (CATS) were also rushed to the schools.

Delhi Chief Minister Arvind Kejriwal, accompanied by Deputy Chief Minister Manish Sisodia, visited Majeedia Hospital and interacted with the children admitted there and enquired about their health.

"The situation is under control. A few students are still complaining of irritation and are being treated. Doctors assured that there was nothing to worry about as they say there is no lasting effect of the gas and all will be fine," Kejriwal told reporters after visiting the hospital.

He said the government has already ordered an inquiry and "action" will be taken against the culprits.

Delhi Deputy Chief Minister Manish Sisodia, who also holds the Education portfolio, separately visited Batra Hospital.

Sisodia said he had spoken to the doctors and the condition of the students was "normal".

He said the District Magistrate has been ordered to probe the leak.

Sisodia announced that schools in the area were ordered shut for the day, and exams postponed.

He said the situation would be reviewed on Monday.

Sisodia later tweeted, "Maximum students have gone back to home. Few are still under observation in hospitals. AIIMS team working on medical impact study."

Earlier in the day BJP leader Vijender Gupta also visited the students in ESI Hospital and accused the authorities of "negligence".

Delhi Commission for Women (DCW) Chief Swati Jaihind also met the students at Batra hospital.

Union Health Minister J.P. Nadda instructed all Centre-run hospitals to be ready to help the victims.