At the end of a weekend that encapsulated the soaring highs and crushing lows of Olympic sport, Great Britain made its first mark on the medal table at its home Games with a silver medal for road cyclist Lizzie Armitstead and a bronze for Rebecca Adlington in the pool.

Team GB's chef de mission, Andy Hunt, said the medals would be "momentum builders" for the rest of the team as they chase their best performance since the 1908 Games in London.

After the disappointment of seeing favourite Mark Cavendish miss out in the men's road race on Saturday, it was appropriate given the sport's success in recent years that it was cycling that gave British fans their first medal to cheer.

While Adlington surrendered her Olympic 400m freestyle title to French swimmer Camille Muffat, she was delighted to secure bronze in a faster time than that which brought her victory four years ago. She qualified only in eighth place for the final and had to reel in her rivals over the final 100m, to huge roars in the Aquatics Centre.

"The crowd were absolutely amazing. There were so many people who wanted me to get gold and I'm sorry I couldn't do that but I'm so pleased I got a bronze," she said.

Earlier, as the rain lashed down on the Mall, organisers were hopeful that Armitstead's silver would begin a run of home success that would build on the buoyant mood established by Danny Boyle's opening ceremony.

But as the first weekend of London's Games came to a close, organisers were again on the back foot over hundreds of empty seats in some venues. London 2012 chairman Lord Coe was forced to backpedal after first saying all venues were "packed to the gunnels".

He later said the problem was being taken seriously, as the organising committee introduced a series of measures to try to deal with the issue – largely caused by members of the "Olympic family" failing to take up seats in accredited areas.

In the face of rising public anger, officials said troops would be invited to fill seats in some venues, while there was also the possibility of inviting other accredited staff to fill gaps.

They have also introduced a Wimbledon-style returns system, so that if spectators leave a session early, their tickets can be recycled to others on the park for a nominal fee. Organisers insisted that there was less of a problem with official sponsors, who were using their tickets.

Still shaking after cycling 140.3km (87 miles) in appalling conditions and narrowly missing out to Dutch former world champion Marianne Vos in a sprint finish, Armitstead said: "It means four years of hard work has paid off. I'm very, very happy to be a medallist and the first one for GreatBritain."

Four riders broke away on the final climb of Box Hill in Surrey and they became three with around 20km to go when the American Shelley Olds suffered a puncture.

On the Mall in teeming rain, spectators huddling under union flag umbrellas roared Armitstead home, but Vos was too strong. The British rider said she had been "praying for rain", knowing she would thrive in the conditions.

Team GB officials had woken to hyperbolic headlines such as "Brits in pieces" after an opening day in which Cavendish and Hannah Miley had failed to meet sky-high expectations in the men's road race and in the pool.

Hunt had appealed for calm and said morale remained strong, reminding the British public there was plenty of time for the team to achieve its goal of more medals in more sports than in longer than a century.

Relieved to be off the mark, he paid tribute to Armitstead and her teammates for their "outstanding accomplishment" and said their success would be a "momentum builder" for the British team, which ended the day in 16th place in the medal table.

The rain did not deter the thousands of spectators who lined the roads, which followed the same route as the men the previous day on a loop through Surrey from the Mall, but with two climbs of Box Hill instead of seven.

"It was something I'll never really forget, not being able to hear myself think. The noise pushed us all the way to the finish," Armitstead said.

Adlington, the golden girl of British swimming in 2008 after surprising the world by winning two gold medals as a 19-year-old, still has her stronger event to come – the 800m freestyle.

There were plenty of other reasons to be cheerful. Among them were encouraging performances from Zara Phillips, competing on her horse High Kingdom at Greenwich Park for the first time, and from the women'sgymnastics team, with Beth Tweddle recording her best score since the Beijing Games.

Ben Ainslie, the sailor who next weekend hopes to secure his fourth consecutive gold medal, also began his campaign in the Finn class off the Dorset coast.

But there was bitter disappointment for marathon runner Paula Radcliffe, whose stellar career has been blighted by Olympic failure, when she confirmed that she was pulling out of the British team with injury.

Aside from the potentially explosive ticketing issue and some relatively minor transport problems, there were other niggles.

Organisers blamed congestion on mobile phone networks caused by excessive use of Twitter for technical problems with the timing system during the cycling road races, and the crowds watching Team GB at Wembley were left hungry, thirsty and frustrated when its concessions were unable to accept credit cards.

A Visa spokesman said: "We understand that Wembley's systems failed and therefore they were only accepting cash at the food and beverage kiosks. This cash only decision was made by Wembley management and not Visa. We are working with the Wembley team to help them fix this as soon as possible."

It emerged on Sunday night there had been further problems last week at Wembley, when a set of keys to the stadium went missing.

A London 2012 spokesman said: "Keys being used by police officers on searches at Wembley stadium were reported missing on 24 July. These were internal keys. Locks have been changed and the security of the venue has not been compromised in any way."

A spokeswoman for the Metropolitan police said: "On the morning of Tuesday 24 July, officers on the Olympic policing operation at Wembley stadium reported that internal security keys, which had been being used by them as part of searches, were missing. Officers attended the venue to assist in a search for the missing items but nothing was found."

But, overall, organisers and politicians were pleased the Games had got off to a good start. They will now face their biggest logistical challenge, when London Bridge station experiences its most congested day of the Games as Londoners return to work.

The cross-country in Greenwich Park will attract crowds of 50,000 and the station is also a hub for spectators streaming to the Olympic Park in Stratford and the shooting events in Woolwich. Commuters are being urged to work from home or use alternative routes.