* El Wafa, El Sharara pipelines shut by protests * Tensions high over future of interim congress * Eastern oil port blockade drags into sixth month By Patrick Markey TUNIS, Feb 13 (Reuters) - Libya's oil output dropped to 460,000 barrels per day on Thursday after protesters shut down pipelines from the El Wafa and El Sharara oilfields in a fresh challenge to Prime Minister Ali Zeidan's fragile government. Libya had restored production at the 340,000 barrel-per-day El Sharara field at the start of the year, in a boost for Zeidan's government as it struggles to end a six-month blockade of oil ports in the east. Armed groups, former rebels and tribes often shut down pipelines or occupy oilfields to make demands on the state as Libya tries to overcome instability nearly three years after a NATO-backed revolt toppled leader Muammar Gaddafi. Protesters partially shut down a pipeline near Zintan leading from El Sharara in southern Libya to the port of Zawiya in western Libya, a spokesman for the National Oil Corporation said on Thursday. "There is a problem with El Wafa and there is some problem with the pipeline at El Sharara. Pipeline capacity is decreased because of protests," the spokesman said. He said national production was 460,000 barrels per day (bpd), compared with 566,939 bpd on Tuesday, according to LANA state news agency. He would not give a detailed breakdown for El Sharara. Output at the field had been at 301,000 barrels per day (bpd) on Wednesday, down from its usual output of 340,000 bpd. On Wednesday, protesters also shut gas and oil pipelines from the El Wafa oilfield, which produces around 30,000 bpd of very light oil condensate. El Wafa feeds gas and condensate to the Mellitah terminal complex, which is jointly operated by Italy's Eni and Libya's National Oil Corp. Demands of protesters were not immediately clear. But tensions are growing over the country's interim General National Congress, whose mandate has officially ended, but whose members have extended its term to guarantee stability. Political blocks in the Congress - the National Forces Alliance and Islamists parties Justice and Construction Party, and the Al Wafaa block, are deadlocked are how to proceed in the country's fragile transition to democracy. Rival brigades of former rebels and militias are loosely aligned with competing factions within the Congress, with some demanding its dissolution and new elections, and others supporting its extension. Armed protesters led by a former anti-Gaddafi rebel have seized three oil ports in eastern Libya since August, cutting off around 600,000 bpd of export capacity, to demand more regional autonomy and a greater share of oil wealth. Negotiations to end that blockade have gone nowhere, and the government has warned it could resort to force to break the protest that has cost the state more than $7 billion in lost oil revenues.