By Marina Lopes

MAPUTO (Reuters) - The deadliest attacks in Mozambique in more than a decade by suspected opposition gunmen have rekindled memories of a 1975-1992 civil war and put pressure on the ruling party to rethink the marginalization of its main political foe.

Four policemen and three civilians were killed in ambushes of a truck and two buses at the weekend - a tactic widely used by guerrillas in the 1980s - raising fears that the mineral-rich southern African nation's two-decade peace may be under threat.

Renamo, the guerrilla movement founded around independence in 1975 with the backing of white-ruled Rhodesia and apartheid South Africa to take on the Marxist Frelimo party which has ruled the country ever since, has denied it attacked civilians.

But the group - which has seats in parliament but is effectively excluded from power in what is a de facto one-party state - is widely suspected. Military chief Paulino Macaringue was quoted as saying the army was awaiting orders from President Armando Guebuza to strike back.

However, several newspaper editorials said that instead of a military clamp-down, Guebuza should offer an olive branch to Renamo leader Afonso Dhlakama, who has been pushed into the political wilderness by Frelimo's stranglehold on politics and the economy since the war ended with a shaky truce.

"The President of the Republic has a vast array of options, that he doesn't use, to come to an agreement with the leader of Renamo and make Renamo feel included in the democratic process," the Magazine Independente said.

Analysts say Renamo is in no shape to launch widespread attacks, with at most 1,000 veteran guerrillas at its disposal and popular support that amounted to only 16 percent of the vote in 2009 elections.

"It is seriously doubtful that these people could actually wage a war," said Joseph Hanlon, a Mozambique analyst at Britain's Open University.

But many Mozambicans are worried that even the slightest violence could upset the relative political stability that has underpinned an unprecedented economic boom based on massive foreign investment in coal mining and natural gas exploration.

The off-shore Rovuma field is believed to hold enough gas to supply Germany, Britain, France and Italy for 15 years, and mining giants Vale and Rio Tinto have invested nearly $10 billion in mines in Tete province, home to some of the world's largest untapped coal deposits.

ELECTION THREAT

The source of the fiercest Frelimo-Renamo tension in over a decade stems from preparations for next year's presidential election and Renamo's thwarted attempts to reduce Frelimo's control of the National Election Commission.

Foreign observers criticized the former Portuguese colony's last two elections as not fair and lacking transparency.

Dhlakama retreated to the Renamo stronghold, the remote Gorongosa Mountains, in October, threatening to open guerrilla training camps.

Renamo leaders vowed to sabotage the polls after their electoral reform drive in parliament failed. Although the presidential election is not until October 2014, voter registration starts next month and municipal polls are due in November.

The security forces responded to those threats last week when police raided Renamo headquarters in the central town of Muxungue, arresting 15 people and tear-gassing bystanders.

The next day, Renamo gunmen killed four policemen and one of their own members died in an assault on the Muxungue police station. Two days later gunmen shot up a gasoline truck and two buses in the same district, killing three people.

Renamo security chief Osufo Madate said the party had finally got fed up with being brushed aside by Frelimo, which controls 191 of the 250 seats in parliament and dominates nearly every aspect of public life in the nation of 23 million.

"If we continue with our peaceful behavior, it will mean the end of us. From now on, whenever we are attacked, we will retaliate adequately," he told Reuters.

Speaking to reporters this week in the Gorongosa Mountains, Dhlakama said he had talked to President Guebuza by telephone but turned down a face-to-face meeting because it "would not result in anything".

Since the ambushes, police have deployed armed escorts for vehicle convoys on the highway near Muxungue - an alarming echo of the civil war on the main road running down the spine of the country.

South African bus company Intercape, one of whose vehicles was attacked, said it had suspended its service along the route.

Worryingly for the mining companies in the northern province of Tete, the Sena line - the only rail link from the coal fields to the Indian Ocean - runs through former Renamo strongholds and was frequently attacked and blown up during the war.

"This is not how the country will attract new investment," Mozambican analyst Fernando Lima said.

"It is not possible for the country to maintain this spiral of growth and foreign investment and have this type of news of armed conflict and photographs of people bleeding."

(Writing by Ed Cropley; Editing by Robin Pomeroy)