Tunisia's Islamist-led coalition has failed to agree with the opposition on how to end weeks of political deadlock, mediators said on Saturday, ahead of planned anti-government protests.

Tunisia was plunged into crisis by the 25 July assassination of opposition MP Mohamed Brahmi, the second of its kind in six months, with both attacks blamed on militant Salafists.

The powerful UGTT trade union, which has been playing the role of mediator, on Saturday presented the government's latest proposals for resolving the crisis to opposition group, the National Salvation Front (NSF).

But after the meeting, Hamma Hammami, a representative of the opposition, said the group had replied to the ruling coalition's proposals and that "the key to ending the crisis is in the (government's) hands."

Hammami refused to elaborate on the proposals, except to confirm reports that they envisaged a change of government by September 29 at the latest, after a month of national dialogue on the new cabinet and the future constitution.

But the NSF, a wide umbrella group of opposition parties, has repeatedly demanded the immediate resignation of the governing coalition, led by the moderate Islamist movement Ennahda.

It has refused to engage in any national dialogue until a non-partisan cabinet has been formed.

An anti-government protest is planned for 1600 GMT in the capital, in the form of a human chain stretching several kilometres (miles) from parliament to the Kasbah, where the government's headquarters are located.

Last Saturday, the NSF launched what it called the "week of departure," a week-long campaign of protests aimed at bringing down the government, starting with a mass rally outside parliament.

But the demonstration attracted fewer people than two similar protests held earlier this month -- 10,000 according to police estimates.

The planned week-long campaign failed to mobilise the kind of Egyptian mass protests that preceded the army's ouster of Islamist president Mohamed Morsi on 3 July.

The opposition has accused Ennahda of failing to rein in Tunisia's jihadist movement, which is widely blamed for the murder of Brahmi and opposition MP Chokri Belaid, another prominent secular politician whose assassination in February brought down the first Islamist-led coalition.

Ennahda has also been accused of mismanaging the economy and failing to improve living standards, with Egypt's Morsi facing similar criticism from protesters ahead of the military coup.