Equatorial Guinea on Sunday accused Spain, France and the United States of "interference" in its presidential and legislative elections scheduled for November 20.

President Teodoro Obiang Nguema Mbasogo, who has ruled his country with an iron fist for 43 years, launched his bid for a sixth term this week in a first campaign event.

Equatorial Guinea, a former Spanish colony, reproached the three countries after their diplomats attended a campaign event this week by one of the two opposition movements authorised to present candidates in the polls.

The foreign ministry described it as "interference in the country's internal affairs" in a statement.

Obiang's dominant Democratic Party of Equatorial Guinea (PDGE) holds 99 of the 100 seats in the outgoing lower house of parliament and all the senate seats.

It was the country's single legal political movement until 1991, when multi-party politics were introduced.

Running against Obiang are Andres Esono Ondo of the Convergence for Social Democracy party (CPDS) and Buenaventura Monsuy Asumu, who represents the Party of the Democratic Social Coalition.

In a tweet on Thursday, US State Department spokesman Ned Price said Washington was "concerned by reports of arrests and harassment of opposition members and civil society" and called on the government to hold "free and fair" elections.

"Equatorial Guinea can cultivate a more inclusive, peaceful, and democratic society by ensuring the expression of diverse political perspectives, a free and fair voting process, and the protection of the human rights of all individuals," Price said.

Security forces have waged a ruthless campaign over several weeks including arresting opponents.

But the government says the detentions are part of a crackdown on a "plot" by the opposition to plan "attacks" on "gas stations, Western embassies and ministers' homes".

Obiang, 80, came to power in a 1979 coup and is the longest-ruling head of state in the world excluding monarchs.

He has never officially been re-elected with less than 93 percent of the vote.

More than 425,000 voters are registered for the polls out of a population of around 1.4 million.

The country possesses major oil and gas resources, but a majority of its 1.3 million people live below the poverty line, according to the World Bank.