The president and vice-president of the Methodist Conference have condemned the shooting of protesters on the Gaza Israel border that left 18 Palestinians dead and hundreds injured.

The Good Friday protests were dubbed the 'Great March of Return' and were designed to highlight the plight of the two million residents penned in to the territory for more than a decade. Israel has accused the ruling Hamas party of orchestrating the march in order to provoke confrontation.

In a statement, Rev Loraine Mellor and Jill Baker appealed to 'all sides to step back urgently from actions that can only lead to more suffering, and to seek peaceful solutions'.

However, they accuse Israel of 'crimes against humanity', saying the killing of unarmed protesters is 'unacceptable in any circumstance'.

'The Government of Israel is bound by the same standards of international law as any other nation. Live rounds must never be used except in the most critical circumstances to protect life,' the Methodist leaders say.

They highlight the 'desperate' situation in Gaza, where according to the UN, clinics and hospitals are reporting a 'huge' rise in malnutrition and 80 commonly-used medicines are not available.

They say: 'Electricity is on for four hours a day and water in many areas of Gaza is undrinkable. Reports of Hamas fighters being armed for further conflict have been greeted with dismay by the civilian population itself.

'We pray for a resolution of the conflict in the region, for the end to occupation and a just resolution for Palestinian refugees.

'We call on the Government of Israel, the Palestinian Authority, and all regional and international powers to do everything possible to prevent a further slide into conflict and bloodshed, to provide humanitarian aid, and to revive the stalled reconstruction of Gaza.'

Catholic aid agency CAFOD also highlighted the effects of Israel's blockade.

Its director of advocacy Neil Thorns said: 'The horror of this tragedy underlines how the political process, along with the economy, are paralysed, and living conditions for the Palestinians have worsened, leaving communities with deepened levels of desperation and frustration.

'The blockade of Gaza, through which Israel controls all but one of the entry points, continues to have a devastating effect, limiting people's movement, their access to health care and markets, and leaving the majority of the population dependent on humanitarian aid. Bearing the brunt of this suffering are the most vulnerable, including the elderly, persons with disabilities, women and children, who have experienced unimaginable suffering.'

He said: 'The only way out of the cycle of violence for Israelis and Palestinians to be truly secure, is to revive hope for a just Israeli-Palestine peace process, aimed at creating the conditions for a stable peace based on justice, where all must be treated equally.'

UN secretary-general Antonio Guterres and the European Union's top diplomat, Federica Mogherini, have called for an independent investigation into Friday's bloodshed.

However, Avigdor Lieberman, the Israeli defence minister, rejected criticism of Israel's actions, saying soldiers along the Gaza frontier 'deserve a medal' and did what was necessary to protect the border.

'As for a commission of inquiry – there won't be one,' he told Israeli Army Radio.