The partnership means that a huge population can now send Twitter messages at standard rates and receive them for free, Twitter said. Twitter so far has activated full SMS service in the U.S., Canada, New Zealand, and the U.K., the company said.

Bharti Airtel said on that the tie-up with Twitter would be exclusive for only four weeks, suggesting that after that period, the service may also be offered by other service providers in India.

Bharti Airtel wants to take advantage of the exclusivity period to ensure that Twitter is associated with its brand by consumers, Bharti Airtel said.

Twitter did not reply to an e-mail asking if it planned ties with other service providers.

SMS access to Twitter is likely to be more popular in India than mobile services that require Internet access, because a very small percentage of mobile phones in India are Internet enabled. Twitter launched a service in India last year that allowed subscribers to use the service without paying international SMS charges, but it discontinued the offering, citing costs, according to some reports.