Rising tension, a planned opposition boycott and fresh memories of a day of deadly clashes hung over Liberia's presidential election run-off on Tuesday, which incumbent leader Ellen Johnson-Sirleaf is poised to win.

Turnout was low in early voting in the capital, Monrovia, which was quiet overnight, and the government shut down two pro-opposition radio stations.

Johnson-Sirleaf is set to secure a second term in the poll, the West African state's first locally organized presidential contest since its civil war, after her rival Winston Tubman withdrew in protest against alleged fraud in the first round.

Tensions spilt over on Monday, when at least one person was shot dead after police broke up a pro-Tubman demonstration. A Liberian police officer was also detained by U.N. peacekeepers after he admitted to firing live rounds during the clash, Liberia's police inspector said.

A Reuters correspondent said people trickled to polling stations on Tuesday compared to the numbers that turned out in the pouring rain in the first round in October.

I feel good, I exercised my right, student Bellevilley Armah said after voting at the William Tubman High School, named after the challenger's uncle, a previous president.

When asked why so few people were lining up to vote, she said: Maybe they are afraid. Maybe they are afraid of what happened yesterday.

The vote is expected to provide a measure of the West African state's progress since civil war ended in 2003 and to pave the way for new investment, but fears are rising it could instead usher in open-ended political turmoil.

Human rights watchdog Amnesty International called for a thorough investigation of Monday's killing and urged restraint through the rest of the election process.

U.S. President Barack Obama called on Liberian security forces to show restraint and allow peaceful protest, and he warned against any voting violations.

Those gains (to consolidate democracy) must not be set back by individuals who seek to disrupt the political process, he said. The international community will hold accountable those who choose to obstruct the democratic process.

Tubman, a Harvard-educated former U.N. ambassador, seized on the clashes to criticise Johnson-Sirleaf.

It shows to you why the Liberian people are determined to get rid of this leader. She is somebody who will use violence against peaceful people, he said.

Two radio stations -- King FM and Love FM -- which are seen to support Tubman and running mate and former football star George Weah were shut down overnight.

Armed policemen came to the station while we were giving news. They put a journalist under gunpoint and asked him to leave. They brought a (court order) based upon the complaints by the justice minister and the minister of communication, Paul Mulbah, station manager for Love FM.

It is unfortunate. We have gone back to the old days.

A spokesman for the government confirmed that they had shut down the radio stations but declined to give further details. The government also called the rally an illegal provocation and urged voters not to be intimidated on election day.

Liberia is one of the world's poorest countries, with over half of its people surviving on less than 50 U.S. cents a day. Fourteen years of intermittent fighting killed nearly a quarter of a million people and has left its infrastructure in ruins.

DON'T WANT TROUBLE

Johnson-Sirleaf took nearly 44 percent of the first round vote on October 11 and has since won the backing of the third-placed finisher, former warlord Prince Johnson.

Tubman -- who won about 33 percent in the first round -- said last week he would withdraw from the race and called for a boycott because of evidence of fraud. He said he would only be willing to participate in a second-round if it were delayed by two to four weeks and counting procedures were amended.

International election observers called the October 11 vote mostly free and fair, and the United States, the United Nations, regional bloc ECOWAS and the African Union have all criticised Tubman's decision.

Many of his supporters are unemployed former fighters.

Johnson-Sirleaf became Africa's first freely elected female head of state in 2005, and has been internationally praised for reducing the country's debt and maintaining peace. But she faces criticism within for the slow pace of development.

Analysts had anticipated that a smooth election would trigger a surge in foreign investment in resources such as iron ore and oil, which have already attracted major firms including ArcelorMittal, BHP Billiton and Anadarko Petroleum.

But many in Liberia are just hoping for a peaceful poll.

We don't want any trouble. But monkey and baboon not getting along, said a Monrovia resident who called himself Tarr. Liberians have nicknamed Johnson-Sirleaf 'monkey' and Tubman 'baboon' and frequently used stuffed animal mascots during campaigning.

(Writing by David Lewis; Editing by Giles Elgood)