* Hong Kong lawmakers divided over protests
* Some lawmakers wear black masks to legislature
* Metro fully opens but to shut early
* Protests planned across city on weekend
* Arrests total more than 2,300
* Annual harbour race cancelled due to protests
* (Adds police blackmail comments, mental health survey)
By Twinnie Siu and Jessie Pang
HONG KONG, Oct 11 (Reuters) - Hundreds of mask-wearing pro-democracy protesters marched through Hong Kong's central business district on Friday, occupying a main thoroughfare and disrupting traffic as the Chinese-ruled city braced for another weekend of unrest.
Chanting their core demands, which include universal suffrage and an independent inquiry into perceived police brutality, the crowd occupied the financial district at lunchtime before peacefully dispersing.
Hong Kong's metro operator opened all stations for the first time in a week ahead of more planned anti-government protests, while the city's legislature began its first session since protesters stormed the building in July.
Pro-establishment and democratic lawmakers shouted at each other before the session, underscoring the tension and divisions in Hong Kong after four months of often violent anti-China protests.
Some lawmakers wore black masks as they sat in the chamber, while others carried placards reading: "Police brutality still exists, how can we have a meeting?"
Hong Kong leader Carrie Lam invoked British colonial-era emergency laws last Friday and banned the wearing of face masks which protesters have used to shield their identities.
The protests have plunged the city, an Asian financial hub, into its worst crisis since it returned to Chinese rule in 1997, posing the biggest popular challenge to Chinese President Xi Jinping since he came to power in 2012.
What began as opposition to a now-withdrawn extradition bill has evolved into a pro-democracy movement fanned by fears that China is stifling Hong Kong's freedoms, guaranteed under a "one country, two systems" formula introduced in 1997.
China denies the accusations and says foreign countries, including Britain and the United States, are fomenting unrest.
The Chinese embassy in Bangkok on Friday condemned Thai politicians for showing support for Hong Kong activists.
"This is wrong and irresponsible. China hopes that relevant people will understand the truth about problems in Hong Kong, act carefully and do useful things for the friendship between China and Thailand," the embassy said in a statement.
Ninety people have been arrested for anti-mask law violations in the past week, pushing the total number of arrests since June to more than 2,300, police said on Friday. Many of those arrested are under 16, authorities said.
Police said they were investigating four reports of blackmail involving emails from a group claiming to be pro-democracy and threatening to target shops unless they fund protests via bitcoin.
"The intimidating messages even include videos of rioters inflicting extensive damage to shops over the past few weeks," said acting police chief superintendent Kong Wing-cheung. Protesters have targeted China banks and shops with links, or perceived links, to mainland China.
Many residents fear the emergency laws may be expanded, further eroding civil liberties, but the government said on Thursday it would not bring in any other measures.
Hong Kong has been relatively calm for a few days after a street march by tens of thousands of people last Sunday spiralled into a night of violent clashes.
Several protests are planned across the city on Friday night and through the weekend. While daytime marches have been large scale, most night protests are small scale and sporadic.
Metro operator MTR Corp, whose network carries about 5 million passengers a day, said the MTR would shut two hours early at 10 p.m. (1400 GMT) on Friday to allow more repairs after stations were torched or trashed last weekend.
The MTR shut down completely last Friday following arson attacks and has operated only partially since then. Protesters have targeted the MTR because it has been blamed for closing stations at the government's behest to contain demonstrations.
Many stores and businesses have shut early to avoid becoming targets, and due to metro closures, putting another burden on the city's faltering economy as it faces its first recession in a decade.
Several major conferences and other events have been called, with the latest being an annual swimming race in the city's famed Victoria Harbour.
The protest movement still appears to have a broad base of support despite the violence and vandalism carried out by small groups of front-line protesters.
Residents, particularly those in their 20s and 30s, are calling for the protection of civil liberties. Many young people are also angry about the city's hugely expensive property, widening inequality and poor job prospects.
The stress of four months of protests has seen the mental health of Hong Kongers deteriorate to its worst level in eight years, media reported.
Hong Kong's mental health score in 2019 was 46.41, the lowest since the annual survey started in 2012, according to the study organised by the Mental Health Month Organising Committee. A healthy score is between 52 and 68. The score was 50.20 last year.
On Oct. 16, Lam is due to deliver the city's annual policy address, which traditionally contains sweeteners and support for business and investment, but is expected to also contain measures to address social issues.
Reporting by Jessie Pang, Felix Tam and Twinnie Siu Writing by Michael Perry and Farah Master; Editing by Frances Kerry and Nick Macfie