Media playback is unsupported on your device Media caption The strike has led to crowded platforms and frustrated commuters

Passengers are enduring overcrowding, delays and chaos on Southern trains as a strike by RMT union members continues.

The 72-hour walkout is due to go on until Friday, despite the RMT advising members to sign new contracts.

Passenger group Transport Focus said commuters were fed up and wanted services to get back to normal.

The RMT said the strike was "rock solid", while Southern said it was running 61% of its normal timetable.

Fresh talks are to be held on Wednesday in a bid to end the dispute.

The RMT has held a series of walkouts since April over the role of conductors on Southern services.

But, Transport Focus spokesman Anthony Smith said passengers had "lost the plot as to what the original rights and wrongs of the dispute actually were".

"It has become so bitter and protracted," he said.

In a letter to RMT general secretary Mick Cash, Southern CEO Charles Horton said: "I'm prepared to free my diary from tomorrow morning onwards to meet and to show your serious intent, I would like the RMT to call off the rest of the strike action planned for this week."

Mr Cash said talks were a golden opportunity to break the deadlock. "The RMT will be at the talks," he said.

Analysis by BBC South East Political Editor Helen Catt

The RMT thinks having new on-board supervisors on the trains instead of conductors will lower safety standards.

Southern says they will be "safety trained", rather than "safety critical" and there is a grey area over the difference.

The two sides don't seem to be coming any closer together.

The row is getting increasingly bitter and it doesn't look as if there is any quick resolution to this.

Image copyright Dan S Kennedy Image caption Hundreds of passengers were unable to fit onto the 07:21 train at Redhill station

Image caption The RMT union has been picketing outside Brighton station

Passengers have been venting their frustration on social media.

Martin Still tweeted: "I've forgotten what they are arguing about and bored with it all now - perhaps just sack everyone, managers and strikers and start again."

Liam Mustapha posted: "Cant get to school today because of @SouthernRailUK strike today. I don't think I or other commuters can take this any longer. #southernfail."

Earlier train services between Brighton and Gatwick Airport via Balcombe were disrupted because of over-running engineering work by Network Rail as a result of a broken down engineers' train.

Russell Woollen said: "@SouthernRailUK why risk doing engineering works before a day like this?? You can't make it up. Appalling."

Image copyright Twittter Image caption Frustrated passengers are making their views known on social media

Southern, which runs trains linking parts of Kent, Surrey, Sussex and Hampshire with London, is introducing driver-only operation and wants conductors to accept new on-board supervisor positions.

The RMT revealed it had received a legal challenge to the strike hours before the start of the walkout, but said it would go ahead while it examined the details.

The union previously advised conductors to accept new contracts to "protect their position" - but said it would seek to overturn them.

Media playback is unsupported on your device Media caption Govia Thameslink boss says changes mean better customer service

Mick Lynch, the assistant secretary general of the RMT, told the BBC: "We're sorry that people in London and the South East have got to put up with this.

"If the company gets round the table and we can hammer out a deal which we think is reasonable, then we can call off these strikes and get everyone back to work."

Strike dates announced by the RMT:

00:01 Tuesday 11 October to 23:59 Thursday 13 October

00:01 Tuesday 18 October to 23:59 Thursday 20 October

00:01 Thursday 3 November to 23:59 Saturday 5 November

00:01 Tuesday 22 November to 23:59 Wednesday 23 November

00:01 Tuesday 6 December to 23:59 Thursday 8 December

Mayor of Seaford Lindsay Freeman said the south coast town's summer tourist trade had dropped and some commuters had given up jobs in London because of the strikes.

Ms Freeman said she was very concerned about the impact of a strike planned for 5 November, when up to 40,000 people travel to bonfire celebrations in the East Sussex town of Lewes.