Former president Nicolas Sarkozy indicated that his right-wing UMP party would probably support Sunday’s march after being invited to meet President François Hollande at the Elysée presidential palace for the first time since he lost the 2012 election on Thursday morning.
Hollande had promised to tighten security in the face of the “development of the threat” following Wednesday's attack, he said after the meeting.
On Wednesday Sarkozy called on the French people to “resist the temptation of amalgams and to present a united front against terrorism, barbarism and murderers”.
Left-wing parties, including Hollande’s Socialists, the Communist Party and the Greens, have already declared their support for the rally.
Marine Le Pen of the far-right Front National on Thursday said that she was waiting for a call from Prime Minister Manuel Valls, judging it “obvious” that a party that won 25 per cent in last year’s European elections should be invited to join.
Valls later told RTL radio that there could be “no exclusion from national unity” but added that national unity involved the values of “tolerance and a rejection of amalgams” and that “the terrorists also want us to identify violence, intolerance and hatred with Islam”.
Rallies in solidarity with the victims took place across France on Wednesday evening.
A manhunt is under way for two brothers, Chérif and Saïd Kouachi, who are suspected of being the killers.