MUMBAI, June 19 (Reuters) - Vodafone Group has sold a bigger-than-planned 18% stake in India's Indus Towers for $1.82 billion on Wednesday, with local telecom operator Bharti Airtel picking up shares to increase its stake to nearly 50%.

UK's Vodafone, which owns a 21.5% stake in mobile-tower operator Indus, had initially planned to sell a 10% stake but strong investor demand promoted it to nearly double the sale size, a banking source aware of the matter said, requesting anonymity as they are not allowed to speak to the media.

Vodafone said it sold 484.7 million Indus shares at 310-341 rupees per share, raising 153 billion rupees, or 1.7 billion euros, in gross proceeds that it will use to repay debt.

It has bank borrowings of 1.8 billion euros against its Indian assets, which also includes a stake in Vodafone Idea , the country's debt-saddled No.3 telecom operator by subscribers.

Indus's stock slid as much as 9.4%, before paring losses to trade down 4.3% at 329.60 rupees around midday in Mumbai, with more than 750 million shares traded, already their busiest-ever session.

Airtel, India's No.2 telecom operator, said it bought a roughly 1% stake in Indus, raising its stake to around 49%. It did not disclose the price.

Vodafone Group now has a 3.1% stake in Indus. Vodafone Idea also has a stake in Indus. Private equity giant KKR and Canadian fund CPPIB sold their entire stakes in February.

Vodafone Group sold its stake via so-called block deals, where investors sell shares in the market. They have risen in popularity in India with the stock market at record-high levels.

Vodafone Idea shares were mostly flat, while Bharti Airtel's shares were down 1%. ($1 = 83.4300 Indian rupees) (Reporting by Sriram Mani in Mumbai and Nandan Mandayam in bengaluru; Editing by Mrigank Dhaniwala and Savio D'Souza)