A Raja and Suresh Kalmadi face investigators in Delhi

A Raja quit last month amid what has been described as India's biggest corruption investigation

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Indian police have begun questioning former Telecommunications Minister A Raja over an alleged multi-billion dollar corruption scandal.

Mr Raja was forced to quit last month over the sale of mobile telephone licences for a fraction of their value.

Police have also raided the homes of Commonwealth Games organiser Suresh Kalmadi in a separate corruption case.

On Wednesday thousands of people rallied in Delhi against alleged abuse involving the ruling Congress party.

Further rallies are planned across the country in the coming weeks.

Investigators are looking into why so-called 2G spectrum phone licences were sold in 2008 for a fraction of their value, costing the government $37bn (£23bn) in lost revenue, according to the national auditor.

Mr Raja, who denies any wrongdoing, was questioned by the Central Bureau of Investigation (CBI) on Friday.

He is a member of the DMK party, an important member of the Congress-led ruling coalition.

The 2G spectrum scandal has prompted the most serious political stand-off in recent times in India.

The winter session of parliament ended this month in deadlock, as the government rejected opposition demands for a top-level inquiry.

'Not running away'

Earlier this week, Indian Prime Minister Manmohan Singh told a Congress party conference he had nothing to hide, and was happy to face a parliamentary panel over the 2G inquiry.

Suresh Kalmadi adopted a defiant tone outside his home after the police search

Congress party president Sonia Gandhi told the same conference that corruption was a disease in India, as she unveiled a five-point action plan to tackle the problem.

Mr Singh has been accused of reacting slowly to the 2G scandal, which is described as the country's biggest ever corruption investigation.

Another high-profile inquiry is continuing into claims that organisers of the Delhi Commonwealth Games swindled millions of dollars from the October event.

On Friday, the CBI searched Games chief organiser Suresh Kalmadi's homes in the capital, Delhi, and in the western town of Pune, and the home of his personal secretary.

After police finished their search, Mr Kalmadi told journalists outside his Delhi home he was innocent until proven guilty.

"I am ready for any investigation on any issue," he said, reports AFP news agency. "I am not running away."

Three other senior officials from the Games organising committee, including its sacked treasurer, were arrested last month.

They are accused of forgery and cheating over the awarding of contracts before October's event.

The Central Vigilance Commission, a national watchdog, has received complaints that up to $1.8bn (£1.1bn) of Games money was misappropriated.