Representatives from Christian Concern and the Christian Legal Centre previously submitted a ten-page document to the Strasbourg-based court, warning that Islamic law is "incompatible" with the Western legal system.
Prior to the proceedings, Christian Concern's Head of Public Policy, Tim Dieppe, said: "We hope that our intervention will help to show the judges that they should not allow Sharia law to overrule a member state's domestic law."
The legal dispute concerns a Greek widow from a Muslim family who claims she is entitled to the estate of her late husband, under Greek law.
But two of her husband's sisters claim that the inheritance should instead be adjudicated by a Muslim legal expert (a mufti), in accordance with sharia law.
Commenting on the Molla Sali v. Greece case, Christian Concern warned that Sharia law is "discriminatory by nature".
It added: "A ruling at this level that sharia law can have supremacy over a member state's domestic law could have dire effects on the Council of Europe's 47 member states, including the United Kingdom.
"Sharia law is fundamentally incompatible with human rights, and our intervention explains that the Universal Islamic Declaration of Human Rights restricts freedom of speech and freedom of religion, as does the Cairo Declaration on Human Rights."