Controlled explosions of suspicious packages and evacuations are taking place across Brussels after the deadly terrorist attacks in the Belgian capital on Tuesday.

The Berlaymont building of the European Commission, the royal palace, the highest building in the Belgian capital and the Etterbeek campus of the Free University of Brussels, were among the evacuated buildings on suspicions of possible further attacks.

The authorities have also detonated suspicious packages found at multiple locations across the capital. A Kalashnikov rifle and an unexploded suicide belt were reportedly found at Zaventem airport.

Staff not essential for the running of Belgium's two nuclear power plants Tihange and Doel have been evacuated as part of safety measures in the aftermath of the attacks.

According to latest reports, the explosions at Zaventem airport and Maelbeek metro station claimed the lives of at least 34 people and left more than hundred other injured. The most likely causes of the explosions were suicide bombings.

At present, Brussels is in a lockdown, with the airport and the entire public transport system closed. The terror alert has been risen to the highest possible fourth level.

International trains to Brussels, including the Eurostar from London and the Thalys from Paris, have also been suspended. According to reports, Zaventem airport will remain closed at least until March 24.

Three days of national mourning have been declared in Brussels and Belgium.

Countries neighbouring Belgium as well as other European states, including Bulgaria, have heightened security measures after the attacks. Sofia airport has stepped up its level of security and a large-scale anti-terror training will take place in the Bulgarian capital in the late hours on Tuesday.

The attacks occurred only several days after the chief suspect in last year's November 13 attacks in Paris, Salah Abdeslam, was detained in a police raid in the Brussels suburb of Molenbeek last week.