Leaders of Afghanistan's three major ethnic minority political parties, all of whom hold senior positions in the government, announced from Turkey on Saturday that they have formed a coalition to save Afghanistan from chaos, issued a list of demands for reforms by President Ashraf Ghani, and vowed to hold mass protests unless they are met.

The stunning development followed weeks of gathering political turmoil and public unrest after a devastating terrorist bombing in the capital on 31 May. It brought together a group of powerful ex-militia leaders, once rivals in a civil war, in an extraordinary alliance that could present Ghani and his shaky government with its most serious challenge since taking office in 2014.

The group's statement was issued from Ankara where Abdul Rashid Dostum, an ethnic Uzbek strongman who is still technically first vice president in the Ghani government, moved recently on grounds of ill health despite being under investigation in Kabul for sexual assault against an elderly political rival. Dostum's aides circulated the statement on social media.

The other leaders -- Muhammad Atta Noor, an ethnic Tajik and provincial Governor; Mohammed Mohaqiq, an ethnic Hazara leader and deputy to the government's chief executive; and Foreign Minister Salahuddin Rabbani, a member of Noor's Jamaat-e-Islami party -- have been visiting Dostum in the past week for a family wedding in the lavish home where he has often lived in periods of exile.

The group, calling itself the Coalition for the Salvation of Afghanistan, said their aim was to "prevent the collapse of the government, avoid chaos and restore public trust." They demanded that Ghani devolve power to cabinet ministries and provinces, stop "overreaching" his authority for personal motives, schedule long-promised elections, and obey the constitution and the law. It also called for Dostum's full authority to be restored and a government attack against him to be investigated.

Ghani's office responded coolly and calmly to the provocative salvo. Presidential spokesman Shahhussain Murtazawi told news outlets that the Government "welcomes any move" that contributes to national interests, but noted that the individuals leading the coalition are "involved in the Government" and thus also "accountable for its shortcomings." If the group has "any alternative plans for overcoming the current situation," he said, "they should share them."

There was no comment from the office of Abdullah Abdullah, the chief executive officer who has been estranged from Ghani for many months. Abdullah, from the Jamaat-e-Islami party, has disappointed party figures such as Noor for making too many concessions to Ghani in an effort to keep the struggling government afloat.

A variety of political figures and observers reacted sceptically to the news, suggesting that the ethnic minority leaders, all of whom have had differences with Ghani while in office, may be less interested in government reforms than in using a period of public anger and unhappiness to press for political advantage. They also noted that Ghani, an ethnic Pashtun, has been criticised for concentrating power in the hands of his ethnic and tribal allies and marginalising other ethnic groups.

A spokesman for Gulbuddin Hekmatyar, the Pashtun former fugitive warlord who returned to Kabul recently in a peace deal with Ghani, said the new coalition seemed "suspicious" and might be more interested in "personal demands" than public ones. "Why have they been silent for so long?" asked the spokesman, Kareem Amin. "You can't be inside the system and criticise it too. "

Hekmatyar, in a separate statement Saturday, called on all Afghans to unite and support the Ghani government at a time of crisis. The country is suffering from high unemployment and a protracted insurgent conflict. The 31 May bombing was a major blow to the nation's confidence in its rulers.

It was unclear whether the ethnic opposition leaders, who have called for city-wide demonstrations starting Monday, would be able to draw much support from the protesters that filled the streets of Kabul for most of June after the huge bombing and several subsequent violent incidents.

The groups erected tent colonies on major streets where speakers demanded change night after night. The tents were dismantled by security forces on 20 June, but protest groups vowed to return to the streets in force after Ramadan and Eid, the Muslim fasting month and holiday that ended this week.

But although many of the protesters' demands were similar to those listed by the ethnic leaders, the composition and tone of their impromptu movement, called "Uprising for Change," was completely different. A mix of students, academics, liberal activists and women's groups, as well as families of bomb victims, they called mainly for justice, security and more responsive governance.

The most strident voice in the new coalition has been that of Noor, a wealthy northern Governor who, until recently, was negotiating with Ghani to obtain a greater share of power. During the fraud-plagued 2014 elections, which both Abdullah and Ghani claimed to have won, Noor threatened to create violent unrest if Ghani was declared the winner.

Last week, delivering a message to a large crowd at the end of Ramadan in the northern city of Mazar-e-Sharif, Noor warned that if Ghani did not meet the group's demands, "we will come by the thousands and thousands to Kabul." Sources in the security community said the group planned to gather its forces in several suburban locations and march to the Presidential Palace.