The al-Wefaq National Islamic Society said in a post on Twitter that the move was a dangerous step toward escalation of tension.
These were political executions that violated the basic legal and ethical norms, the group said.
It added that the Manama regime is complicit with Saudi Arabia in committing this crime.
Al-Wefaq further offered condolences to the people of Bahrain and the families of the two martyrs.
The rights group Americans for Democracy & Human Rights in Bahrain (ADHRB) also denounced the executions and described the two Bahraini martyrs as victims of torture.
On Monday, Riyadh announced the execution of two Bahrainis named Jaafar Muhammad Sultan and Sadiq Majeed Thamer, charged with “joining a terrorist cell aimed at destabilizing the security” of Saudi Arabia and Bahrain.
This brought the total number of executions in the kingdom to nine this month.
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