By Helen Murphy
BOGOTA (Reuters) - Colombian President Juan Manuel Santos' public approval rating has plunged to its lowest level since he took office in 2010, a poll released on Wednesday showed, due to a farmers' strike marred by violence and scant progress in peace talks with Marxist rebels.
Just 21 percent of Colombians polled in late August and early September said they had a positive opinion of Santos, down from 48 percent in a similar survey conducted at the end of June, according to the Gallup poll.
The survey was conducted in the middle of a two-week farmers' strike during which media images showed riot police wearing armor confronting workers dressed in ponchos.
Santos, 62, was ridiculed during the early days of the strike for saying it was non-existent and that everything was normal.
The ongoing dispute, in which farmers have blocked roads to snarl transportation nationwide, has put pressure on the center-right president, who has until November to decide whether to run for a second term.
Farmers complain that free trade accords with the United States and Europe have made it impossible for them to compete with imports, which they say are now less expensive.
The poll - carried out between August 27 and September 2. - showed 72 percent of those surveyed had a negative image of Santos, compared with 44 percent in June. Some 7 percent had no opinion.
Santos took office with an approval rating of 74 percent and maintained decent ratings through the beginning of peace talks with the Revolutionary Armed Forces of Colombia, or FARC.
The government has spent nearly 10 months working through a tough agenda in Cuba with rebel leaders.
Details of the negotiations have not been made public, but many Colombians are concerned that Santos has offered too many concessions to the FARC leadership.
Meanwhile, the rebels have continued to kill scores of soldiers and attack oil and mining installations, putting additional pressure on Santos to clinch a deal.
Santos' popularity also has been hit by slowing economic growth and social problems such as poverty and unemployment, especially in rural areas where some communities support the FARC, which is considered a terrorist group by Europe and the United States.
Gallup polled 1,200 people in the five largest urban areas of the country. The survey had a margin of error of 3 percentage points.
(Editing by Andrew Cawthorne and Xavier Briand)