Egypt's second-largest Islamist group, the Nour Party, has claimed the Muslim Brotherhood urged it to attack opponents of Mohamed Morsi outside the presidential palace in December 2012.
The remarks are likely to further strain ties between the once close allies.
Nour deputy head Nader Bakkar claimed on Sunday that his party had rejected calls by Brotherhood deputy leader Khairat El-Shater to join the group's supporters in forcibly breaking up a sit-in by Morsi opponents.
At least ten were killed in clashes outside the presidential palace when thousands protested against a constitutional decree granting Morsi sweeping new powers.
Morsi, along with 14 other Islamists and Brotherhood leaders, are being tried for inciting the killing and torturing of opponents during the clashes.
Brotherhood leaders allegedly summoned civilian supporters to attack opponents camping outside the palace, and deadly clashes between both sides ensued.
This was the "first fracture" between the allies, with the Brotherhood at the time accusing the Salafist group of "treason," according to Bakkar.
The Brotherhood was first to instigate infighting between civilians, he told an interviewer on the privately-owned Al-Hayat channel.
The Salafist leader said his party would say this in court, another blow to the movement of toppled president Morsi.
Bakkar had previously slammed the Islamist group for embracing "violence" and "defamation."
The Nour Party was the only Islamist group to support Morsi's ouster and the political roadmap that followed, which envisions an amended constitution by early next year, with parliamentary and presidential votes to follow by mid-2014.
Nour has said it will campaign for a Yes vote in the constitutional referendum to bring stability, further infuriating Morsi loyalists.
Nour Party head Younis Makhyoun has urged a halt to street protests by Brotherhood supporters to help Egypt progress in its democratic transition.
Morsi, who was toppled by the army in July after millions protested against him, also faces charges related to his escape from prison during the 2011 uprising.
Authorities have launched a sustained crackdown on Islamists, killing hundreds and arresting thousands, including much of the Brotherhood leadership.