The Belgian capital was on lockdown Tuesday following deadly explosions at Brussels airport and on a metro train which killed more than 30 people and wounded over 200. 

Belgium hiked its terror threat to its highest level after the carnage wreaked by the early morning blasts, just days after the capture of key Paris attacks suspect Salah Abdeslam in the heart of the city.

The transport networks were shut down, with trams, buses and trains brought to a standstill and the European Union quarter completely sealed off by police. 

"If in Brussels, stay where you are and do not travel without reason. Stay inside," the Belgian national crisis centre said on its website.

The chaotic scenes in Europe's capital recalled the days in the wake of the November Paris attacks, when Brussels was put on lockdown for five days as officials warned of an imminent threat as they searched for key suspect Abdeslam.

The European Commission, the EU's executive arm, said it was urging employees to stay at home or remain indoors.

About 25,000 European civil servants work in Brussels, many just hundreds of metres (yards) from the metro attack where around 20 people were killed and over 100 wounded.

Brussels authorities also urged employees already at work to remain inside throughout the course of the work day.

The landmark Grand Place was deserted with several museums and other top tourist destinations in the city's historic centre closed.

The government also asked residents to refrain from making phone calls and to communicate via text message and instant messages so as not to overload communications systems.

"Please everyone, prioritise communication over WiFi (WhatsApp, Facebook, Twitter etc)," said Telecoms Minister Alexander De Croo in a tweet.

Around 30,000 students were sent home from the capital's main universities, reversing an earlier decision to let classes go ahead. 

Schools were open but the education ministry urged administrators not to allow children to go home unaccompanied.

Major shopping malls were shuttered and would be at least until the end of Tuesday, Belga news agency reported.

Low-cost shopping chain Colruyt said its stores remained open but that it was monitoring the situation closely.

 Belgian royals deny that Brussels palace evacuated

 Belgium's royal palace in central Brussels was not evacuated on Tuesday following a bomb blast at a nearby metro station and two explosions at the city's airport, the palace said in an official twitter message.

"Contrary to some reports, the royal palace has not been evacuated," the twitter message said.

Earlier broadcaster RTBF had reported King Philippe and Queen Mathilde were in shock and the palace had been evacuated.      

 FBI coordinating with Belgium counterparts after Brussels attacks

 Top US military and Justice Department officials have been briefed on the attacks in Brussels and the FBI has begun coordinating with its Belgium counterparts and other 
US agencies following the blasts, US officials said on Tuesday.

US Defense Secretary Ash Carter and General Joseph Dunford, the chairman of the Joint Chiefs of Staff, were briefed, officials said. The two were due to testify before a congressional committee later on Tuesday on the Pentagon's budget.

Attorney General Loretta Lynch was also briefed on the explosions, according to a Justice Department official, who said the department and the FBI are also coordinating efforts across the U.S. government and with Belgium authorities.