President Vladimir Putin signed the "alliance and strategic partnership" agreement which formalises Russian dominance of the tiny separatist region's foreign policy, armed forces and economy, as well as placing Russian guards on the border with the rest of Georgia.
The signature "is a step towards a de facto annexation of the Abkhazia region by the Russian Federation" and a "violation of the principles of Georgia's territorial integrity and international law," Georgian Foreign Minister Tamar Beruchashvili told journalists.
More than 30,000 opposition supporters protested in Tbilisi this month against the planned deal, which comes amid a diplomatic firestorm over Russia's involvement in a separatist uprising in eastern Ukraine and annexation of Crimea last March.
Abkhaz opposition parties have also voiced fears that the agreement undermines their tiny would-be country's bid for real independence.
More From This Section
Some provisions of the agreement "may have undesirable consequences for the sovereignty" of Abkhazia, the Council of Abkhazia's Public Chamber said on Friday.
The deal that Putin signed after talks with Abkhaz President Raul Khajimba in the Black Sea resort of Sochi says Moscow and Sukhumi will unite armed forces and jointly guard Abkhazia's border with Georgia.
The agreement, which says it is prompted by "new global and regional challenges and threats," specifies that any attack against Abkhazia will be viewed as aggression against Russia, which will provide military aid.
Putin also said that Russia would give Abkhazia five billion rubles (USD 111 million) in 2015.
"We have set new tasks and long-range goals. They are backed up with serious financing," Putin said, quoted by Interfax news agency.
Abkhazia broke away from Georgia following a civil war in the 1990s. The war between Moscow-backed separatists and Tbilisi forced some 250,000 residents to flee their homes, most of them ethnic Georgians.