(Corrects first name of president in paragraph 12)
* Renamo has vowed to disrupt road, rail traffic
* Government blames Renamo guerrillas for bus attack
* Foreign mining investment threatened by unrest
By Marina Lopes
MAPUTO, June 21 (Reuters) - Gunmen killed two people in ambushes on vehicles in Mozambique on Friday, two days after the opposition Renamo party threatened to sabotage transport routes in the mineral-rich southern African country.
Just before the attacks, police arrested Renamo information chief Jeronimo Malagueta, who on Wednesday had announced that the ex-guerrilla group would halt traffic on main roads and the Sena railway linking the northwest coal-fields to the sea.
Persistent tension between Renamo and the ruling Frelimo party, who fought each other in a 1975-92 civil war, has alarmed citizens and investors just as the former Portuguese colony enjoys a boom driven by bumper coal and gas discoveries.
"We urge all Mozambicans to stay vigilant to premeditated and spontaneous attacks and threats to public safety," Interior ministry spokesman Pedro Cossa told a news conference in Maputo.
Cossa said a truck driver and his passenger were killed and five others wounded in Friday's attacks. He denied reports that a bridge was damaged in the central province of Sofala, a Renamo stronghold.
Malagueta was detained in the early hours of Friday, the government and Renamo said. A Renamo spokesman urged its backers to mass outside the prison in Maputo where he was being held.
Eleven soldiers and policemen and five civilians have been killed since April in attacks blamed on Renamo, which was founded around the time of Mozambique's independence from Portugal in 1975 with the help of white-ruled Rhodesia and apartheid South Africa to counter the Marxist Frelimo.
Renamo did not claim responsibility for Friday's ambushes but said they were in line with a strategy to pressure Frelimo into relaxing its 20-year stranglehold on politics and the economy.
The line from the city of Tete to the Indian Ocean port of Beira is used predominantly by Brazil's Vale and London-listed Rio Tinto, who are investing billions of dollars in the Tete coalfields.
The track runs close to Renamo's civil war headquarters in the remote Gorongosa mountains and was frequently attacked during the conflict, in which one million people died.
Even though Renamo is in no position to initiate a widespread guerrilla campaign, it is estimated to have 1,000 men under arms and analysts say it could cause enough trouble to upset the foreign mining investment boom.
Foremost among Renamo's complaints is that Frelimo has stacked the election commission in its favour to ensure another landslide victory for President Armando Guebuza's party in a vote due in the second half of 2014.
The two sides have held two months of talks but made no headway.
For many of Mozambique's 23 million people, the most serious Frelimo-Renamo friction in more than a decade has rekindled dark memories of the war, which left the country in ruins.
"This is totally unacceptable. We appeal for everything to stop immediately," said Bishop Dinis Sengulane, a mediator in talks that paved the way for a 1992 peace agreement.
"Dialogue must continue but in a more serious way. Arms were never considered a substitute for dialogue. These arms are trying to suffocate our peace process," he told Reuters.
On the streets of the capital Maputo, 700 km (400 miles) south of the trouble spots, the mood was grim.
"It is very sad because this is definitely a path towards war," said Arlino Gimo, a street vendor. "We feel this is just the beginning. We lived in war for 16 years. We do not want another one. Renamo and Frelimo need to have dialogue." (Writing by Ed Cropley; Editing by Angus MacSwan/Mark Heinrich)