Attanayake has been arrested on charges of forgery, said police spokesman Ajith Rohana.
Attanayake was arrested by Colombo crime division for further questioning in connection with the forged document he produced during the January 8 presidential polls, police said.
The former health minister had produced the document as a pact between President Sirisena and Prime Minister Ranil Wickremesinghe.
The pact pledged to grant Tamil people more powers than accorded by the 13th amendment of the constitution.
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The pact also pledged to reduce national defence expenditure by 40 per cent and to cooperate with the international war crimes investigation against Rajapaksa.
He claimed the document is aimed at compromising national security to win over Tamil support for Sirisena.
Attanayake, the former general secretary of the main opposition United National Party, left the UNP to join former president Mahinda Rajapaksa ahead of the presidential polls.
Sirisena and Wickremesinghe both complained their signatures had been forged by Attanayake.