Josep Maria Castellà, constitutional law professor at the University of Barcelona, has told the BBC the process of Spain enacting direct rule would take less than a week.

"Article 155 allows for many possibilities, with the limit being a total suspension of autonomy," he explains.

"Other options include the suspension of certain competencies, such as security. The important thing for the state will be to ensure control of ports, airports, communication centres and borders by forces of the Spanish state."

Image source, EPA

Spanish PM Mariano Rajoy has said he will take all measures necessary to ensure any declaration of independence has no practical effect.

This is the procedure the Spanish government would follow if it was to trigger Article 155 of the Spanish Constitution to suspend Catalonia’s autonomy:

1. Mr Rajoy would convene an extraordinary Cabinet meeting either Tuesday evening or first thing in the morning
2. If the Cabinet decides to trigger Article 155 the PM will send a formal letter to Mr Puigdemont giving him between 24 and 48 hours to stop acting unconstitutionally
3. If the Catalan leader doesn’t comply, or doesn’t answer, the Cabinet will meet to approve a plan to suspend Catalonia’s autonomy and intervene directly. The plan must detail what actions the central government intends to take.That will be submitted to the Senate, the upper house of Spain’s parliament, where Mr Rajoy's party has a majority
4. The Senate will meet todebate the plan and then vote. If approved by a majority then Catalonia’s autonomy will be suspended under Article 155 - a drastic step than has never been taken before.

Speculation is rife about the reasons Mr Puigdemont's address was delayed at the very last minute:

• Some Spanish sources say there is a possibility of international mediation - something the Catalan leader has previously called for
• Others are reporting that his party's coalition partners, the left-wing CUP party, were not happy with the text of his speech
• A third possibility being discussed is that national politicians from PM Mariano Rajoy's conservative PP and the centrist Ciudadanos have tried to have his parliament appearance cancelled

The BBC's Gavin Lee gives us a sense of how many people have turned out to watch the speech outside the Catalan parliament.

Farmers have driven tractors decorated with the Catalan separatist flag into Barcelona to show their support for independence.

Image source, AFP

The BBC's Tom Burridge explains on the possible next moves for separatists and for Spain's government after the disputed referendum.

The BBC's Guy Hedgecoe, who is in Barcelona, wonders whether the postponement of Mr Puigdemont's speech could indicate a significant change of position.

We're hearing that the Catalan leader's address has been put back, reportedly while party organisers hold a meeting.

The International Monetary Fund has described the Catalan situation as "worrying" during a news conference in Washington DC.

Maurice Obstfeld, chief economist at the IMF, said it "causes a lot of uncertainty for both the Catalan and Spanish economies."

He said that both sides should "not act hastily, and negotiate. There are many potential benefits for both parties if they do."

Catalonia's future is of particular interest in Scotland, which held its own independence referendum in 2014 - the difference being that it was legal under UK law.

Speaking ahead of Mr Puigdemont's much-anticipated address, Scottish First Minister Nicola Sturgeon called for dialogue and said it was "time for the Spanish government to sit down with the government of Catalonia".

She said: "It is time for them to talk and find a way forward – a way forward that respects the rule of law, yes, but a way forward that also respects democracy and respects the right of the people of Catalonia to determine their own future.”

Image source, PA

Is Catalan independence unstoppable? What are the options for Spain? Read our BBC News analysis of the situation.

Image source, EPA/Getty Images

Photographers jostled to get the perfect picture of Mr Puigdemont as he arrived at parliament just a short while ago.

Image source, Getty Images

European Council President Donald Tusk is among those to have urged Mr Puigdemont not to declare independence. In his latest comments on the crisis, the EU leader said the consequences would be "obviously be bad for the Catalans, for Spain and for the whole of Europe".

"Let us always look for what unites us and not for what divides us. This is what will decide the future of our continent."

Image source, Reu

Catalan leader Carles Puigdemont has faced growing pressure not to declare independence - including from influential Barcelona Mayor Ada Colau. Speaking on Monday, she called for de-escalation.

With only a short time to go, the Catalan leader has arrived at the region's parliament where he will give an address. BBC News producer Maria Byrne is outside the building where supporters have gathered.

It is unclear whether Mr Puigdemont will follow through on his threat to make a unilateral declaration of independence. At a news conference in Barcelona earlier, the spokesman for Catalonia's regional government, Jordi Turull, revealed little about what Mr Puigdemont intends to say, but instead hit out at the Spanish government and lauded Catalonia's prosperity.

Our correspondents are following the latest developments in Barcelona and we'll also get the reaction in Madrid.

The BBC's Maria Byrne has been photographing pro-independence crowds in Barcelona.

Mr Puigdemont's address, which is due at 18:00 local time (16:00 GMT), comes after a vote on 1 October, which Catalan officials say resulted in almost 90% of voters backing independence. Turnout was put at 43%.

The vote was deemed illegal by Madrid and suspended by Spain's Constitutional Court. "No" voters largely boycotted the ballot and there were several reports of irregularities. National police were involved in violent scenes as they manhandled voters.