Peruvians on Sunday voted overwhelmingly in favour of constitutional reform aimed at rooting out the corruption that is corroding the country's political institutions and judiciary.

According to initial results, 85 per cent voted in favour of the reforms put forward by popular President Martin Vizcarra, which include banning the immediate re-election of lawmakers.

Despite solid economic growth, Peru has lurched from one political crisis to another over the past two decades.

Four former presidents are being investigated as part of the continent-wide graft scandal involving the Brazilian construction giant Odebrecht.

They include Alejandro Toledo (2001-06), Ollanta Humala (2011-16) and Pedro Pablo Kuczynski (2016-18) as well as numerous other top politicians.

Odebrecht has admitted to paying bribes worth nearly 800 million dollars in 12 countries, 10 of them in Latin America, in order get lucrative state contracts.

The scandal has set off a political avalanche in Peru over the past year.

Vizcarra, who was vice president, took power in March after his predecessor Pedro Pablo Kuczynski stepped down to avoid impeachment over corruption allegations he denied.

At the end of October, Keiko Fujimori, the powerful far-right opposition leader and daughter of former authoritarian president Alberto Fujimori, was jailed preventively for three years while money-laundering charges against her are probed.

And last month Alan Garcia, who served as president from 1985 to 1990 and again from 2006 to 2011, tried to circumvent an Odebrecht-related corruption probe by taking refuge at the Uruguayan embassy. However Montevideo refused to grant him political asylum.

Voters on Sunday approved three of four reforms put forward, including tightening controls on political party financing and introducing changes to the way judges and prosecutors are elected.

It is not yet clear when the reforms will be implemented.

A fourth reform to re-introduce a bicameral parliament was rejected on Vizcarra's recommendation.

His government withdrew its support for the plan after Congress rejected a quota that would have increased women's parliamentary representation.

Congress only approved the referendum in October after Vizcarra threatened to dissolve it unless it joined his popular anti-graft campaign.

"Today is an important day for our democracy," Vizcarra wrote on Twitter earlier.

The referendum was monitored by more than 50 observers from 16 countries in Latin America and Asia, according to the government.