The looming showdown comes three weeks after jihadist attacks in Barcelona, the capital of Catalonia, and a nearby seaside resort.
The looming showdown comes three weeks after jihadist attacks in Barcelona, the capital of Catalonia, and a nearby seaside resort that killed 16 people and wounded more than 120 others.
A man stood near the Catalan parliament on Wednesday morning holding a large sign in English, French and German reading "Freedom for Catalonia".
Nearby another small group of men held another sign against the referendum which accused it of being an attempt to "cheat democracy", an expression used by Prime Minister Mariano Rajoy on Monday to dismiss the planned plebiscite.
Inside the parliament pro-separatist lawmakers, who control the regional assembly in the wealthy northeastern region, are expected to pass the bill, ignoring a ruling by Spain's Constitutional Court that deemed the vote unconstitutional.
They made a request to introduce the bill in the assembly just after 9am (0700) and by 10:30 were still debating wether it should be admitted.
The bill is expected to be voted on during a special urgent session with little debate, and then top Catalan government officials will swiftly sign it.
Rajoy has vowed to immediately challenge the law in Spain's Constitutional Court. His government has also threatened legal action against top Catalan political figures involved in the plebiscite.
The president of the Catalan parliament, Carme Forcadell, said in a tweet that she had requested that the judges of Spain's Constitutional Court be disqualified, calling them "another extension of the state which has lost all legitimacy."
Catalonia, a region of 7.5 million people with its own language and culture that accounts for about one-fifth of Spain's economic output, has significant powers over matters such as education, healthcare and welfare.
But Spain's economic doldrums and a perception that the region pays more in taxes than it receives in investments and transfers from Madrid have helped push the cause of secession from the fringes of Catalan politics to centre-stage.
Adding to the rise in separatist sentiment has been a 2010 ruling by Spain's Constitutional Court which struck down parts of a 2006 autonomy charter which granted new powers to Catalonia and recognised it as "a nation".
Lawmakers who back independence won an absolute majority in the 135-seat Catalan regional parliament for the first time in a September 2015 election. The government that emerged from that vote vowed to begin the process of breaking away from Spain.
Rajoy responded by promising new investments in Catalonia and regularly sent his deputy to the region, but made no significant reforms regarding the division of powers that addressed Catalan concerns.
Madrid could have defused the rising separatist tide had it offered Catalonia a new financing deal a few years ago, said Caroline Gray, an expert on Spanish independence movements at Britain's Aston University.
"If some sort of deal had happened in the past, I personally think we wouldn't be where we are today," she told AFP.
Opinion polls show Catalans are evenly divided on independence. But a majority, over 70 percent, want a referendum to take place to settle the matter.
"I hope they will let us vote. We want a better Catalonia, with more resources and we feel Madrid limits us," Ramon Sanmartin, a 67-year-old retired engineer, told AFP outside the Catalan parliament.
Rajoy steadfastly refuses to let Catalonia hold a plebiscite similar to Scotland's 2014 referendum on independence from Britain, which was approved by London and resulted in a "no" vote.
His conservative Popular Party and the court argue that the Spanish constitution does not allow regions to unilaterally decide on sovereignty.
The head of the opinion section of top-selling Spanish daily El Pais accused Catalan separatists of "intolerance", using Catalan media to "brainwash" the public and of "pressuring" opponents of independence.
He compared their actions to the former dictatorship of General Francisco Franco, who banned the Catalan language.