The alleged assault took place late Monday, when the Sudanese men aged 23 and 31, were at a bus stop with two Russian women. The group became involved in an exchange of words with two young local men, which then erupted into a fight.
The police arrested the two locals near the scene and had 72 hours to decide on whether any further action should be taken.
The incident is the latest of its kind to cast a shadow over the city, about 500 kilometers (310 miles) south of Moscow, which has traditionally been a popular destination for undergraduates to come to study in Russia. European universities send their students for their year abroad or shorter courses, whereas students from Africa and South America often come for longer to train for good degrees at relatively low prices.
However, a string of attacks on foreigners, often attributed to racial motives, have made headlines recently.
On October 9, an 18-year-old architecture student from Peru died after he and his three companions were attacked by about 15 Russian youths. The student was stabbed to death and two of his friends - a fellow Peruvian and a Spaniard - were severely injured.
On October 17, Voronezh State University Albanian student Zami Altin told police that he had been beaten up, and had his mobile phone and ID papers stolen near his dormitory.
The Russian Education and Science Ministry said later it would review the list of colleges recommended to foreign students.
