Senegal's Constitutional Council on Thursday ruled that parliament's unprecedented postponement of the February 25 presidential vote was not in line with the constitution, pitching the country into a new phase of electoral uncertainty.

Opposition presidential candidates and lawmakers last week filed a number of legal challenges to the bill that delayed the vote to December and extended President Macky Sall's mandate in what critics said amounted to an "institutional coup."

The standoff has fuelled widespread unrest and raised international concerns that one of the remaining democracies in coup-hit West Africa is under threat.

In the latest twist, the Constitutional Council decided that "the (postponement) law ... is contrary to the constitution," according to its minutes.

The council also ruled to cancel a decree announced by Sall ahead of the vote that helped set the postponement in motion.

The presidency and government did not reply to requests for comment on whether they would accept the ruling.

All eyes are now on Sall, who said he delayed the election due to a dispute over the candidate list and alleged corruption within the Constitutional Council that would undermine the credibility of the poll.

The council did not specify when the election should be held, which means it is up to Sall to set a reasonable date if he abides by the council's decision, said presidential opposition candidate Anta Babacar, who welcomed the ruling as "good news."

"What matters to a candidate like myself: from April 2 Mr. President Macky Sall is no longer the president, so this election must take place between February 25 and April 2," she told Reuters by phone, referring to the original last date of Sall's mandate in April.

There are only 10 days left before the original poll date and most candidates have not been campaigning since Sall issued his decree on February 3, hours before campaigns were meant to kick off.

The council's ruling has further complicated the electoral outlook, said ruling party lawmaker Abdou Mbow on national television, warning that it would "exacerbate the crisis between the legislature and judiciary."

Any rejection of the decision risks further fanning public outcry which has already triggered clashes between protesters and police that killed three people last Friday and Saturday across several cities.

In the wake of the ruling, it was not immediately clear if organisers would go ahead with protests against the election delay that were planned for Friday and Saturday.

Anta Babacar said she expected tensions would lessen if Sall or authorities did not challenge the council's decision, but "if he doesn't accept it, we are never too far."