Mohamed Nour, president of the recently formed Rally for Democracy and Liberty (RDL), also told Reuters in a telephone interview on Thursday that an attack by his forces on a Chadian border town last Sunday was a purely tactical move.
"We only used less then one sixth of our troops in that attack," he said in his first interview since the fighting over the town of Adre on Chad's eastern border with Sudan.
Chad's government says it is firmly in control of Adre after repelling two attacks. Deby has dismissed the army deserters and rebels who oppose him as "adventurers".
Nour said a major anti-government attack would happen in the near future, "probably in the coming few days", but declined to say where they planned to strike.
"It could even be on N'Djamena," Nour said, referring to the Chadian capital.
The fighting over Adre has raised tensions in the neighbouring region of Darfur where Sudanese fighters have fought Sudan's central government for almost three years.
Fighting in eastern Chad and
Darfur is a growing concern
The Chadian government says its army pursued the RDL forces over the border into Sudan and destroyed their bases there.
It accused Sudan of supporting the attackers and said around 300 of them were killed.
Nour, 35, said the RDL was formed four months ago and had bases in Darfur.
He is from the Tama tribe, which spans both sides of the border, and took part in the rebellion that ousted Hissene Habre as president and brought Deby to power in 1990.
But he said Deby had become corrupt and worse than Habre while in power, so he decided to move into armed opposition. "Then I came to Darfur and set up military camps," he said.
Deby, a former army chief, has been credited with bringing a measure of stability to Africa's newest oil producer, although he has long had a tense relationship with the military.
Nour said the RDL wanted to organise free elections after a two-year interim period, but that he would not stand for office.
Chad has a history of instability
abetted by foreign interference
Scores of Chadian soldiers deserted their barracks in late September before regrouping near the border and joining up with other rebels to attack the government.
Sudan denies backing the Chadian rebels but witnesses in the Sudanese town of el-Geneina, about 30km from the border, said fighters from the Adre attack were in its hospital.
Kofi Annan, the UN secretary-general, has said that the region may be descending into anarchy.