A migrant talks to the media in front of Hungarian riot police at the border crossing with Serbia in Roszke, Hungary September 16, 2015. [Photo/Agencies]

SID, Serbia/ROSZKE - Hungary on Wednesday detained 29 people including a "terrorist" as migrants demanding to be let through the country's newly shut EU frontier clashed with riot police firing water cannon and tear gas while refugees searched for new ways to enter the bloc.

Hungary's decision this week to shut the EU's external border with Serbia was the most forceful attempt yet by a European country to reduce the flood of refugees and economic migrants overwhelming the bloc.

As thousands of migrants scattered across the Balkan peninsula tried to reach the EU, Hungary's prime minister said his country planned to put up a fence along parts of its border with Croatia and on the frontier with Romania to stem the flow.

Helmeted riot police backed by armored vehicles took up positions at the barricaded border crossing with Serbia, where male migrant youths pelted them with stones, demanding entry.

Hungary said it detained a "terrorist" among 29 migrants held during the clashes. At least 20 policemen and two children were injured, a Hungarian security official said.

"Police also captured an identified terrorist," Gyorgy Bakondi, a security adviser to Hungarian Prime Minister Viktor Orban, told state television M1. A government spokesman said the man was "in the database of security services".