With tensions rising, his Sinn Fein party warned it could review its support for police in the British province, where thousands of people died during decades of Catholic-Protestant unrest.
The government rejected accusations that the arrest of Adams, the public face of the republican movement, was politically motivated.
Adams, 65, presented himself at a police station late Wednesday and was arrested for questioning over the murder of Jean McConville, a widowed mother of 10, in 1972.
Detectives were yesterday granted an extra 48 hours until Sunday night to question Adams, after the previous deadline to charge or release him expired, the Police Service of Northern Ireland (PSNI) said.
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Deputy First Minister Martin McGuinness, the top Sinn Fein figure in the power-sharing government in Belfast, blamed "an embittered rump of the old RUC (Royal Ulster Constabulary)" for his continued detention.
The widely-criticised RUC was the police force in Northern Ireland from 1922 to 2001.
"Allegations contained in books and newspaper articles which the PSNI are presenting to Gerry as evidence that he was in the IRA in the 1970s have been around for 40 years," he told the rally.
"This is a replay of the failed effort in 1978 to charge Gerry with membership (of the IRA)."
"Sinn Fein's negotiations strategy succeeded in achieving new policing arrangements, but we always knew that there remained within the PSNI an embittered rump of the old RUC.
"These people want to settle old scores, whatever the political cost.