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 several 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 fueled 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 had set the postponement in motion.

The presidency and government did not reply to requests for comment.

What happens next is unclear. Sall, who is not standing in the vote and has reached the constitutional limit of two terms in power, said he delayed the election because of a dispute over the candidate list and alleged corruption within the Constitutional Council.

In its decision, the council did not specify when the election should be held. There are 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 decision has "left a window of opportunity open for discussion," said Ali Ngouille Ndiaye, a former minister and opposition candidate, on local radio.