A new march by migrants trying to reach Europe overland from Turkey was blocked by police outside Istanbul Monday.

Around 700 mostly Syrian men, women and children from a group that had been blocked for the past week at Istanbul's main bus station set out overnight on foot for the northwestern city of Edirne, 250 kilometers (150 miles) away.

Bulgaria began on Thursday deploying up to 1,000 troops to buttress its southeastern border with Turkey as hundreds of migrants remained stranded for a third day on the Turkish side, a senior official said.

"We have triggered a plan for the start of the step-by-step deployment of up to 1,000 troops along the whole Bulgarian-Turkish border (within the next week)," interior ministry chief of staff Georgy Kostov said.