The president of Tanzania has pledged never to evict the Masai people after an international outcry against plans to turn their ancestral land into a commercial hunting ground for Arab royalty.

Last week the Guardian revealed claims by Masai activists that the government had reintroduced plans to forcibly relocate 40,000 pastoralists to make way for a luxury hunting and safari company based in the United Arab Emirates.

But, as an online campaign gathered steam, President Jakaya Kikwete tweeted: “There has never been, nor will there ever be, any plan by the government of Tanzania to evict the Masai people from their ancestral land.”

The promise was hailed by the Avaaz global activist group after a two-year fight during which 2.3 million people signed a petition against the proposal. In the past week, according to Avaaz, 18,000 people have written to Tanzanian embassies to raise their concerns.

“This is a massive breakthrough,” said Sam Barratt, Avaaz campaign director. “For the first time in 20 years, a Tanzanian president has definitively said the Masai are safe on their land. Over 2 million people around the world have stood arm in arm with the Masai to keep foreign hunters at bay.”

Masai representatives also welcomed the announcement but struck a note of caution. Ole Kulinga, an elder and traditional leader from Loliondo, the affected district, said: “Without our land, we are nothing and this commitment from the president lets us all breathe a sigh of relief. But hunters want this land more than anything and we will only feel safe when we have permanent rights to our land in writing.”

Samwell Nangire, coordinator of the local Ngonett civil society group, noted that Kikwete had tweeted that there had never been a plan to evict the Masai and insisted this was not true. “He should have said we had the plan but we dropped the plan,” Nangire told Associated Press. “The plan was there for sure. But he said there was no plan. He should put in writing the commitment. That is what everyone is waiting for.”

Nangire has said Masai community leaders rejected an offer of 1bn Tanzanian shillings (£369,350) in compensation for the 580 square miles (1,500 sq km) area bordering the Serengeti national park. He estimated that 80,000 pastoralists, whose livestock graze on the land, would have been directly or indirectly affected.

Tanzania had previously rejected claims that it had ambitions to turn the land in Loliondo into a big game hunting reserve for the Ortelo Business Corporation (OBC), a safari company set up by a UAE official close to the royal family. Lazaro Nyalandu, the natural resources and tourism minister, said the “government has no such plans and never entertained the idea of evicting the Masai”.

But Avaaz claims the government “brutally evicted” some Masai communities to make way for a hunting concession run by the OBC in 2009. Officials also promised to shelve the plan last year, the group claims, only to reactivate it recently.

The land in question is an immense plain dotted with acacia trees and watering holes. More than 2 million animals migrate north from Serengeti into Kenya’s adjacent Masai Mara reserve every year.

Kikwete will step down as president next year after two terms in office.