By Aleksandar Vasovic BELGRADE (Reuters) -Thousands of protesters, mainly students, descended on Serbia's capital on Friday ahead of a planned weekend of massive anti-government rallies, many travelling hundreds of miles on foot or by bike.

In a sign of the potential for unrest, President Alexandar Vucic's outgoing prime minister, who has already submitted his resignation over the protests, made a veiled threat to use force to suppress rallies expected to be among the biggest for years.

Students who have been marching or cycling toward Belgrade for days are expected to arrive on Friday evening for a festival-like welcome in the city. Previous student-led rallies