The statement followed a rare general meeting of the legal association that discussed threats to the independence of the judiciary and adopted the resolution.
Bar association president Wijedasa Rajapaksa, who is no relation to the president, said the body held a general meeting Saturday for the first time in nearly 24 years.
The association joined local and international opposition to the impeachment, which comes amid allegations that the attempt is politically motivated. The United States has expressed concern over the move and asked Sri Lanka to refrain from infringing judicial independence. Opposition politicians and the country's powerful Buddhist monks have all opposed the move.
Sri Lanka's government has been accused of undermining the judiciary and concentrating power in President Rajapaksa's hands.
The bar association represents Sri Lanka's lawyers, with some 13,000 members. They last met in 1988 when a lawyer was killed in police custody.
Government-allied parties have leveled charges of hidden wealth and misuse of power against Chief Justice Shirani Bandaranayake.
They came after Bandaranayake ruled that a proposed bill was against the constitution.
Bandaranayake has denied the charges against her.
The bar association has also asked for the participation of two observers— retired chief judges or supreme court judges—at a parliamentary inquiry if the government opts to continue with its plan.
In July, a mob allegedly instigated by a government minister attacked a court house in the northern Mannar district because they disagreed with a verdict.
Last month, Majnula Tillakaratne, a judge responsible for transfers and disciplinary action against judges, was attacked days after he said unnamed powerful people were trying to interfere with his work and were threatening judges and their families.