The writer, the 2001 Nobel Peace Prize laureate and a former secretary general of the United Nations, is the joint special envoy for Syria of the United Nations and the Arab League.

A group of influential countries from the U.N. Security Council and the Middle East will meet Saturday in Geneva to agree on an action plan for peace in Syria.

The situation could hardly be more grave. Since last spring, many thousands of Syrians have risen up to demand change. While at first they gathered peacefully, in the face of appalling government brutality some have resorted to arms. Others, especially members of minorities, have sat on the fence or supported the government, and they fear the alternative.

The resulting maelstrom has shocked the world. Battles have raged through city after city. Whole neighborhoods have been shelled into ruins. Families have been massacred. Thousands have been killed and thousands more detained, while hundreds of thousands have fled their homes. Many civilians are trapped in combat zones, not receiving medical care or humanitarian aid.

Violence has reached the capital, Damascus, and has spilled over to neighboring states. And as the chaos deepens, terrorist elements have sought to exploit it.

In March, everyone agreed to a six-point plan that provided a ladder the parties could climb down and a mechanism, the U.N. Supervision Mission in Syria, to help them sustain a cease-fire so political negotiations could start.

But that plan has not been implemented. After an initial lull, the violence got worse. Syria’s government, which bears the biggest responsibility, continues to use extreme violence against both unarmed and armed protesters. For its part, the opposition lacks unity and some elements have intensified their attacks against government forces and installations.

Unarmed U.N. observers have had to suspend their activities. They remain at their posts, ready to reengage if the parties show the political will. The Security Council will soon decide on the mission’s future.

This conflict is among Syrians, and they must be the ones who solve it. But it would be naive to think they could, on their own, end the violence now and enter into a meaningful political process.

Many external powers are deeply involved. Despite formal unity behind the six-point plan, mutual mistrust has made them work at cross-purposes. Intentionally or otherwise, they have encouraged the government and parts of the opposition to believe that force is the only option. This serves no one’s interest — least of all that of the Syrian people.

It is time for all who have influence on the parties, and all who bear responsibility for international peace and security, to act positively for peace. With the support of the secretaries general of the United Nations and the Arab League, I have asked participants in Saturday’s meeting to form an action group whose members will work together until peace is achieved.

The participants include those with influence on the Syrian government and its opposition. Members must commit to act in unison to end the bloodshed and implement the six-point plan, avoiding further militarization of the conflict. It is abundantly clear that the violence will not stop without joint, sustained pressure from those with influence, including consequences for noncompliance.

But something more is essential. I expect all who attend Saturday’s meeting to agree that a Syrian-led transition process must be achieved in accordance with clear principles and guidelines.

There must be a democratic and pluralistic future for Syria that complies with international standards on human rights and protects the rights of all communities.

This must include a government of national unity that would exercise full executive powers. This government could include members of the present government and the opposition and other groups, but those whose continued presence and participation would undermine the credibility of the transition and jeopardize stability and reconciliation would be excluded.

The transition would also include a meaningful national dialogue and a constitutional revision subject to popular approval, followed by free and fair multiparty elections. Stability and calm must be ensured throughout by functioning institutions and protection of all groups within Syria’s diverse society. There must be a commitment to accountability and to national reconciliation.

There is no substitute for the hard work of helping the Syrians forge their own political future, in full respect of Syria’s sovereignty, unity and territorial integrity. The international community has long agreed that any transition must be led by Syrians. We must come together to help the Syrian people embrace and achieve this future through peaceful means.

If all participants in Saturday’s meeting are ready to act accordingly, we can turn the tide of violence and embark on a road to peace in which the Syrian people determine their future. If not, the downward spiral will continue — and may soon become irreversible.